diff --git a/api/.gitignore b/api/.gitignore
new file mode 100644
index 0000000..9e6a643
--- /dev/null
+++ b/api/.gitignore
@@ -0,0 +1,11 @@
+.idea/
+.vscode/
+/coverage/
+/vendor/
+/logs/*
+!/logs/README.md
+.phpunit.result.cache
+/composer.lock
+/codeception.yml
+
+/tests/
diff --git a/api/Dockerfile b/api/Dockerfile
new file mode 100644
index 0000000..8707838
--- /dev/null
+++ b/api/Dockerfile
@@ -0,0 +1,5 @@
+FROM php:8.0-apache
+
+WORKDIR /var/www
+
+RUN a2enmod rewrite
\ No newline at end of file
diff --git a/api/app/dependencies.php b/api/app/dependencies.php
new file mode 100644
index 0000000..ec98059
--- /dev/null
+++ b/api/app/dependencies.php
@@ -0,0 +1,29 @@
+<?php
+declare(strict_types=1);
+
+use TorstenHettstedt\TimekeepingApi\Application\Settings\SettingsInterface;
+use DI\ContainerBuilder;
+use Monolog\Handler\StreamHandler;
+use Monolog\Logger;
+use Monolog\Processor\UidProcessor;
+use Psr\Container\ContainerInterface;
+use Psr\Log\LoggerInterface;
+
+return function (ContainerBuilder $containerBuilder) {
+    $containerBuilder->addDefinitions([
+        LoggerInterface::class => function (ContainerInterface $c) {
+            $settings = $c->get(SettingsInterface::class);
+
+            $loggerSettings = $settings->get('logger');
+            $logger = new Logger($loggerSettings['name']);
+
+            $processor = new UidProcessor();
+            $logger->pushProcessor($processor);
+
+            $handler = new StreamHandler($loggerSettings['path'], $loggerSettings['level']);
+            $logger->pushHandler($handler);
+
+            return $logger;
+        },
+    ]);
+};
diff --git a/api/app/middleware.php b/api/app/middleware.php
new file mode 100644
index 0000000..780c46f
--- /dev/null
+++ b/api/app/middleware.php
@@ -0,0 +1,9 @@
+<?php
+declare(strict_types=1);
+
+use TorstenHettstedt\TimekeepingApi\Application\Middleware\SessionMiddleware;
+use Slim\App;
+
+return function (App $app) {
+    $app->add(SessionMiddleware::class);
+};
diff --git a/api/app/repositories.php b/api/app/repositories.php
new file mode 100644
index 0000000..b3d4996
--- /dev/null
+++ b/api/app/repositories.php
@@ -0,0 +1,13 @@
+<?php
+declare(strict_types=1);
+
+use TorstenHettstedt\TimekeepingApi\Domain\User\UserRepository;
+use TorstenHettstedt\TimekeepingApi\Infrastructure\Persistence\User\InMemoryUserRepository;
+use DI\ContainerBuilder;
+
+return function (ContainerBuilder $containerBuilder) {
+    // Here we map our UserRepository interface to its in memory implementation
+    $containerBuilder->addDefinitions([
+        UserRepository::class => \DI\autowire(InMemoryUserRepository::class),
+    ]);
+};
diff --git a/api/app/routes.php b/api/app/routes.php
new file mode 100644
index 0000000..fc279da
--- /dev/null
+++ b/api/app/routes.php
@@ -0,0 +1,26 @@
+<?php
+declare(strict_types=1);
+
+use TorstenHettstedt\TimekeepingApi\Application\Actions\User\ListUsersAction;
+use TorstenHettstedt\TimekeepingApi\Application\Actions\User\ViewUserAction;
+use Psr\Http\Message\ResponseInterface as Response;
+use Psr\Http\Message\ServerRequestInterface as Request;
+use Slim\App;
+use Slim\Interfaces\RouteCollectorProxyInterface as Group;
+
+return function (App $app) {
+    $app->options('/{routes:.*}', function (Request $request, Response $response) {
+        // CORS Pre-Flight OPTIONS Request Handler
+        return $response;
+    });
+
+    $app->get('/', function (Request $request, Response $response) {
+        $response->getBody()->write('Hello world!');
+        return $response;
+    });
+
+    $app->group('/users', function (Group $group) {
+        $group->get('', ListUsersAction::class);
+        $group->get('/{id}', ViewUserAction::class);
+    });
+};
diff --git a/api/app/settings.php b/api/app/settings.php
new file mode 100644
index 0000000..2ad3504
--- /dev/null
+++ b/api/app/settings.php
@@ -0,0 +1,26 @@
+<?php
+declare(strict_types=1);
+
+use TorstenHettstedt\TimekeepingApi\Application\Settings\Settings;
+use TorstenHettstedt\TimekeepingApi\Application\Settings\SettingsInterface;
+use DI\ContainerBuilder;
+use Monolog\Logger;
+
+return function (ContainerBuilder $containerBuilder) {
+
+    // Global Settings Object
+    $containerBuilder->addDefinitions([
+        SettingsInterface::class => function () {
+            return new Settings([
+                'displayErrorDetails' => true, // Should be set to false in production
+                'logError'            => false,
+                'logErrorDetails'     => false,
+                'logger' => [
+                    'name' => 'slim-app',
+                    'path' => isset($_ENV['docker']) ? 'php://stdout' : __DIR__ . '/../logs/app.log',
+                    'level' => Logger::DEBUG,
+                ],
+            ]);
+        }
+    ]);
+};
diff --git a/api/composer.json b/api/composer.json
new file mode 100644
index 0000000..babb684
--- /dev/null
+++ b/api/composer.json
@@ -0,0 +1,42 @@
+{
+    "name": "torsten-hettstedt/timekeeping-api",
+    "description": "REST-Api zum F\u00fcllen einer Tabelle f\u00fcr die Arbeitszeiterfassung.",
+    "keywords": [
+        "microframework",
+        "rest",
+        "router",
+        "psr7"
+    ],
+    "license": "GPL-2.0-or-later",
+    "authors": [
+        {
+            "name": "Torsten Lücke",
+            "email": "arbeit@torsten-hettstedt.de",
+            "homepage": "http://torsten-hettstedt.de/"
+        }
+    ],
+    "require": {
+        "php": "^8.0",
+        "monolog/monolog": "^2.2",
+        "php-di/php-di": "^6.3",
+        "slim/psr7": "^1.3",
+        "slim/slim": "^4.7",
+        "ext-json": "*"
+    },
+    "require-dev": {
+        "phpstan/phpstan": "^0.12.80",
+        "codeception/codeception": "^4.1.18",
+        "codeception/module-phpbrowser": "^1.0.0",
+        "codeception/module-asserts": "^1.0.0"
+    },
+    "autoload": {
+        "psr-4": {
+            "TorstenHettstedt\\TimekeepingApi\\": "src/"
+        }
+    },
+    "autoload-dev": {
+        "psr-4": {
+            "TorstenHettstedt\\TimekeepingApi\\Tests\\Unit\\": "tests/unit"
+        }
+    }
+}
\ No newline at end of file
diff --git a/api/html/.htaccess b/api/html/.htaccess
new file mode 100644
index 0000000..f5d1969
--- /dev/null
+++ b/api/html/.htaccess
@@ -0,0 +1,21 @@
+<IfModule mod_rewrite.c>
+  RewriteEngine On
+
+  # Some hosts may require you to use the `RewriteBase` directive.
+  # Determine the RewriteBase automatically and set it as environment variable.
+  # If you are using Apache aliases to do mass virtual hosting or installed the
+  # project in a subdirectory, the base path will be prepended to allow proper
+  # resolution of the index.php file and to redirect to the correct URI. It will
+  # work in environments without path prefix as well, providing a safe, one-size
+  # fits all solution. But as you do not need it in this case, you can comment
+  # the following 2 lines to eliminate the overhead.
+  RewriteCond %{REQUEST_URI}::$1 ^(/.+)/(.*)::\2$
+  RewriteRule ^(.*) - [E=BASE:%1]
+  
+  # If the above doesn't work you might need to set the `RewriteBase` directive manually, it should be the
+  # absolute physical path to the directory that contains this htaccess file.
+  # RewriteBase /
+
+  RewriteCond %{REQUEST_FILENAME} !-f
+  RewriteRule ^ index.php [QSA,L]
+</IfModule>
diff --git a/api/html/index.php b/api/html/index.php
new file mode 100644
index 0000000..8b4f272
--- /dev/null
+++ b/api/html/index.php
@@ -0,0 +1,78 @@
+<?php
+declare(strict_types=1);
+
+use TorstenHettstedt\TimekeepingApi\Application\Handlers\HttpErrorHandler;
+use TorstenHettstedt\TimekeepingApi\Application\Handlers\ShutdownHandler;
+use TorstenHettstedt\TimekeepingApi\Application\ResponseEmitter\ResponseEmitter;
+use TorstenHettstedt\TimekeepingApi\Application\Settings\SettingsInterface;
+use DI\ContainerBuilder;
+use Slim\Factory\AppFactory;
+use Slim\Factory\ServerRequestCreatorFactory;
+
+require __DIR__ . '/../vendor/autoload.php';
+
+// Instantiate PHP-DI ContainerBuilder
+$containerBuilder = new ContainerBuilder();
+
+if (false) { // Should be set to true in production
+	$containerBuilder->enableCompilation(__DIR__ . '/../var/cache');
+}
+
+// Set up settings
+$settings = require __DIR__ . '/../app/settings.php';
+$settings($containerBuilder);
+
+// Set up dependencies
+$dependencies = require __DIR__ . '/../app/dependencies.php';
+$dependencies($containerBuilder);
+
+// Set up repositories
+$repositories = require __DIR__ . '/../app/repositories.php';
+$repositories($containerBuilder);
+
+// Build PHP-DI Container instance
+$container = $containerBuilder->build();
+
+// Instantiate the app
+AppFactory::setContainer($container);
+$app = AppFactory::create();
+$callableResolver = $app->getCallableResolver();
+
+// Register middleware
+$middleware = require __DIR__ . '/../app/middleware.php';
+$middleware($app);
+
+// Register routes
+$routes = require __DIR__ . '/../app/routes.php';
+$routes($app);
+
+/** @var SettingsInterface $settings */
+$settings = $container->get(SettingsInterface::class);
+
+$displayErrorDetails = $settings->get('displayErrorDetails');
+$logError = $settings->get('logError');
+$logErrorDetails = $settings->get('logErrorDetails');
+
+// Create Request object from globals
+$serverRequestCreator = ServerRequestCreatorFactory::create();
+$request = $serverRequestCreator->createServerRequestFromGlobals();
+
+// Create Error Handler
+$responseFactory = $app->getResponseFactory();
+$errorHandler = new HttpErrorHandler($callableResolver, $responseFactory);
+
+// Create Shutdown Handler
+$shutdownHandler = new ShutdownHandler($request, $errorHandler, $displayErrorDetails);
+register_shutdown_function($shutdownHandler);
+
+// Add Routing Middleware
+$app->addRoutingMiddleware();
+
+// Add Error Middleware
+$errorMiddleware = $app->addErrorMiddleware($displayErrorDetails, $logError, $logErrorDetails);
+$errorMiddleware->setDefaultErrorHandler($errorHandler);
+
+// Run App & Emit Response
+$response = $app->handle($request);
+$responseEmitter = new ResponseEmitter();
+$responseEmitter->emit($response);
diff --git a/api/logs/README.md b/api/logs/README.md
new file mode 100644
index 0000000..d4a602e
--- /dev/null
+++ b/api/logs/README.md
@@ -0,0 +1 @@
+Your Slim Framework application's log files will be written to this directory.
diff --git a/api/phpstan.neon.dist b/api/phpstan.neon.dist
new file mode 100644
index 0000000..45876ad
--- /dev/null
+++ b/api/phpstan.neon.dist
@@ -0,0 +1,2 @@
+parameters:
+  level: 4
diff --git a/api/src/Application/Actions/Action.php b/api/src/Application/Actions/Action.php
new file mode 100644
index 0000000..90b0806
--- /dev/null
+++ b/api/src/Application/Actions/Action.php
@@ -0,0 +1,125 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Application\Actions;
+
+use TorstenHettstedt\TimekeepingApi\Domain\DomainException\DomainRecordNotFoundException;
+use Psr\Http\Message\ResponseInterface as Response;
+use Psr\Http\Message\ServerRequestInterface as Request;
+use Psr\Log\LoggerInterface;
+use Slim\Exception\HttpBadRequestException;
+use Slim\Exception\HttpNotFoundException;
+
+abstract class Action
+{
+    /**
+     * @var LoggerInterface
+     */
+    protected $logger;
+
+    /**
+     * @var Request
+     */
+    protected $request;
+
+    /**
+     * @var Response
+     */
+    protected $response;
+
+    /**
+     * @var array
+     */
+    protected $args;
+
+    /**
+     * @param LoggerInterface $logger
+     */
+    public function __construct(LoggerInterface $logger)
+    {
+        $this->logger = $logger;
+    }
+
+    /**
+     * @param Request $request
+     * @param Response $response
+     * @param array $args
+     * @return Response
+     * @throws HttpNotFoundException
+     * @throws HttpBadRequestException
+     */
+    public function __invoke(Request $request, Response $response, array $args): Response
+    {
+        $this->request = $request;
+        $this->response = $response;
+        $this->args = $args;
+
+        try {
+            return $this->action();
+        } catch (DomainRecordNotFoundException $e) {
+            throw new HttpNotFoundException($this->request, $e->getMessage());
+        }
+    }
+
+    /**
+     * @return Response
+     * @throws DomainRecordNotFoundException
+     * @throws HttpBadRequestException
+     */
+    abstract protected function action(): Response;
+
+    /**
+     * @return array|object
+     * @throws HttpBadRequestException
+     */
+    protected function getFormData()
+    {
+        $input = json_decode(file_get_contents('php://input'));
+
+        if (json_last_error() !== JSON_ERROR_NONE) {
+            throw new HttpBadRequestException($this->request, 'Malformed JSON input.');
+        }
+
+        return $input;
+    }
+
+    /**
+     * @param  string $name
+     * @return mixed
+     * @throws HttpBadRequestException
+     */
+    protected function resolveArg(string $name)
+    {
+        if (!isset($this->args[$name])) {
+            throw new HttpBadRequestException($this->request, "Could not resolve argument `{$name}`.");
+        }
+
+        return $this->args[$name];
+    }
+
+    /**
+     * @param array|object|null $data
+     * @param int $statusCode
+     * @return Response
+     */
+    protected function respondWithData($data = null, int $statusCode = 200): Response
+    {
+        $payload = new ActionPayload($statusCode, $data);
+
+        return $this->respond($payload);
+    }
+
+    /**
+     * @param ActionPayload $payload
+     * @return Response
+     */
+    protected function respond(ActionPayload $payload): Response
+    {
+        $json = json_encode($payload, JSON_PRETTY_PRINT);
+        $this->response->getBody()->write($json);
+
+        return $this->response
+                    ->withHeader('Content-Type', 'application/json')
+                    ->withStatus($payload->getStatusCode());
+    }
+}
diff --git a/api/src/Application/Actions/ActionError.php b/api/src/Application/Actions/ActionError.php
new file mode 100644
index 0000000..2a326db
--- /dev/null
+++ b/api/src/Application/Actions/ActionError.php
@@ -0,0 +1,88 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Application\Actions;
+
+use JsonSerializable;
+
+class ActionError implements JsonSerializable
+{
+    public const BAD_REQUEST = 'BAD_REQUEST';
+    public const INSUFFICIENT_PRIVILEGES = 'INSUFFICIENT_PRIVILEGES';
+    public const NOT_ALLOWED = 'NOT_ALLOWED';
+    public const NOT_IMPLEMENTED = 'NOT_IMPLEMENTED';
+    public const RESOURCE_NOT_FOUND = 'RESOURCE_NOT_FOUND';
+    public const SERVER_ERROR = 'SERVER_ERROR';
+    public const UNAUTHENTICATED = 'UNAUTHENTICATED';
+    public const VALIDATION_ERROR = 'VALIDATION_ERROR';
+    public const VERIFICATION_ERROR = 'VERIFICATION_ERROR';
+
+    /**
+     * @var string
+     */
+    private $type;
+
+    /**
+     * @var string
+     */
+    private $description;
+
+    /**
+     * @param string        $type
+     * @param string|null   $description
+     */
+    public function __construct(string $type, ?string $description)
+    {
+        $this->type = $type;
+        $this->description = $description;
+    }
+
+    /**
+     * @return string
+     */
+    public function getType(): string
+    {
+        return $this->type;
+    }
+
+    /**
+     * @param string $type
+     * @return self
+     */
+    public function setType(string $type): self
+    {
+        $this->type = $type;
+        return $this;
+    }
+
+    /**
+     * @return string
+     */
+    public function getDescription(): string
+    {
+        return $this->description;
+    }
+
+    /**
+     * @param string|null $description
+     * @return self
+     */
+    public function setDescription(?string $description = null): self
+    {
+        $this->description = $description;
+        return $this;
+    }
+
+    /**
+     * @return array
+     */
+    public function jsonSerialize()
+    {
+        $payload = [
+            'type' => $this->type,
+            'description' => $this->description,
+        ];
+
+        return $payload;
+    }
+}
diff --git a/api/src/Application/Actions/ActionPayload.php b/api/src/Application/Actions/ActionPayload.php
new file mode 100644
index 0000000..bf29e73
--- /dev/null
+++ b/api/src/Application/Actions/ActionPayload.php
@@ -0,0 +1,81 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Application\Actions;
+
+use JsonSerializable;
+
+class ActionPayload implements JsonSerializable
+{
+    /**
+     * @var int
+     */
+    private $statusCode;
+
+    /**
+     * @var array|object|null
+     */
+    private $data;
+
+    /**
+     * @var ActionError|null
+     */
+    private $error;
+
+    /**
+     * @param int                   $statusCode
+     * @param array|object|null     $data
+     * @param ActionError|null      $error
+     */
+    public function __construct(
+        int $statusCode = 200,
+        $data = null,
+        ?ActionError $error = null
+    ) {
+        $this->statusCode = $statusCode;
+        $this->data = $data;
+        $this->error = $error;
+    }
+
+    /**
+     * @return int
+     */
+    public function getStatusCode(): int
+    {
+        return $this->statusCode;
+    }
+
+    /**
+     * @return array|null|object
+     */
+    public function getData()
+    {
+        return $this->data;
+    }
+
+    /**
+     * @return ActionError|null
+     */
+    public function getError(): ?ActionError
+    {
+        return $this->error;
+    }
+
+    /**
+     * @return array
+     */
+    public function jsonSerialize()
+    {
+        $payload = [
+            'statusCode' => $this->statusCode,
+        ];
+
+        if ($this->data !== null) {
+            $payload['data'] = $this->data;
+        } elseif ($this->error !== null) {
+            $payload['error'] = $this->error;
+        }
+
+        return $payload;
+    }
+}
diff --git a/api/src/Application/Actions/User/ListUsersAction.php b/api/src/Application/Actions/User/ListUsersAction.php
new file mode 100644
index 0000000..395fcd0
--- /dev/null
+++ b/api/src/Application/Actions/User/ListUsersAction.php
@@ -0,0 +1,21 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Application\Actions\User;
+
+use Psr\Http\Message\ResponseInterface as Response;
+
+class ListUsersAction extends UserAction
+{
+    /**
+     * {@inheritdoc}
+     */
+    protected function action(): Response
+    {
+        $users = $this->userRepository->findAll();
+
+        $this->logger->info("Users list was viewed.");
+
+        return $this->respondWithData($users);
+    }
+}
diff --git a/api/src/Application/Actions/User/UserAction.php b/api/src/Application/Actions/User/UserAction.php
new file mode 100644
index 0000000..b225511
--- /dev/null
+++ b/api/src/Application/Actions/User/UserAction.php
@@ -0,0 +1,27 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Application\Actions\User;
+
+use TorstenHettstedt\TimekeepingApi\Application\Actions\Action;
+use TorstenHettstedt\TimekeepingApi\Domain\User\UserRepository;
+use Psr\Log\LoggerInterface;
+
+abstract class UserAction extends Action
+{
+    /**
+     * @var UserRepository
+     */
+    protected $userRepository;
+
+    /**
+     * @param LoggerInterface $logger
+     * @param UserRepository $userRepository
+     */
+    public function __construct(LoggerInterface $logger,
+                                UserRepository $userRepository
+    ) {
+        parent::__construct($logger);
+        $this->userRepository = $userRepository;
+    }
+}
diff --git a/api/src/Application/Actions/User/ViewUserAction.php b/api/src/Application/Actions/User/ViewUserAction.php
new file mode 100644
index 0000000..f5b2d58
--- /dev/null
+++ b/api/src/Application/Actions/User/ViewUserAction.php
@@ -0,0 +1,22 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Application\Actions\User;
+
+use Psr\Http\Message\ResponseInterface as Response;
+
+class ViewUserAction extends UserAction
+{
+    /**
+     * {@inheritdoc}
+     */
+    protected function action(): Response
+    {
+        $userId = (int) $this->resolveArg('id');
+        $user = $this->userRepository->findUserOfId($userId);
+
+        $this->logger->info("User of id `${userId}` was viewed.");
+
+        return $this->respondWithData($user);
+    }
+}
diff --git a/api/src/Application/Handlers/HttpErrorHandler.php b/api/src/Application/Handlers/HttpErrorHandler.php
new file mode 100644
index 0000000..d7982e5
--- /dev/null
+++ b/api/src/Application/Handlers/HttpErrorHandler.php
@@ -0,0 +1,69 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Application\Handlers;
+
+use TorstenHettstedt\TimekeepingApi\Application\Actions\ActionError;
+use TorstenHettstedt\TimekeepingApi\Application\Actions\ActionPayload;
+use Exception;
+use Psr\Http\Message\ResponseInterface as Response;
+use Slim\Exception\HttpBadRequestException;
+use Slim\Exception\HttpException;
+use Slim\Exception\HttpForbiddenException;
+use Slim\Exception\HttpMethodNotAllowedException;
+use Slim\Exception\HttpNotFoundException;
+use Slim\Exception\HttpNotImplementedException;
+use Slim\Exception\HttpUnauthorizedException;
+use Slim\Handlers\ErrorHandler as SlimErrorHandler;
+use Throwable;
+
+class HttpErrorHandler extends SlimErrorHandler
+{
+    /**
+     * @inheritdoc
+     */
+    protected function respond(): Response
+    {
+        $exception = $this->exception;
+        $statusCode = 500;
+        $error = new ActionError(
+            ActionError::SERVER_ERROR,
+            'An internal error has occurred while processing your request.'
+        );
+
+        if ($exception instanceof HttpException) {
+            $statusCode = $exception->getCode();
+            $error->setDescription($exception->getMessage());
+
+            if ($exception instanceof HttpNotFoundException) {
+                $error->setType(ActionError::RESOURCE_NOT_FOUND);
+            } elseif ($exception instanceof HttpMethodNotAllowedException) {
+                $error->setType(ActionError::NOT_ALLOWED);
+            } elseif ($exception instanceof HttpUnauthorizedException) {
+                $error->setType(ActionError::UNAUTHENTICATED);
+            } elseif ($exception instanceof HttpForbiddenException) {
+                $error->setType(ActionError::INSUFFICIENT_PRIVILEGES);
+            } elseif ($exception instanceof HttpBadRequestException) {
+                $error->setType(ActionError::BAD_REQUEST);
+            } elseif ($exception instanceof HttpNotImplementedException) {
+                $error->setType(ActionError::NOT_IMPLEMENTED);
+            }
+        }
+
+        if (
+            !($exception instanceof HttpException)
+            && $exception instanceof Throwable
+            && $this->displayErrorDetails
+        ) {
+            $error->setDescription($exception->getMessage());
+        }
+
+        $payload = new ActionPayload($statusCode, null, $error);
+        $encodedPayload = json_encode($payload, JSON_PRETTY_PRINT);
+
+        $response = $this->responseFactory->createResponse($statusCode);
+        $response->getBody()->write($encodedPayload);
+
+        return $response->withHeader('Content-Type', 'application/json');
+    }
+}
diff --git a/api/src/Application/Handlers/ShutdownHandler.php b/api/src/Application/Handlers/ShutdownHandler.php
new file mode 100644
index 0000000..f81a71b
--- /dev/null
+++ b/api/src/Application/Handlers/ShutdownHandler.php
@@ -0,0 +1,83 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Application\Handlers;
+
+use TorstenHettstedt\TimekeepingApi\Application\ResponseEmitter\ResponseEmitter;
+use Psr\Http\Message\ServerRequestInterface as Request;
+use Slim\Exception\HttpInternalServerErrorException;
+
+class ShutdownHandler
+{
+    /**
+     * @var Request
+     */
+    private $request;
+
+    /**
+     * @var HttpErrorHandler
+     */
+    private $errorHandler;
+
+    /**
+     * @var bool
+     */
+    private $displayErrorDetails;
+
+    /**
+     * ShutdownHandler constructor.
+     *
+     * @param Request           $request
+     * @param HttpErrorHandler  $errorHandler
+     * @param bool              $displayErrorDetails
+     */
+    public function __construct(
+        Request $request,
+        HttpErrorHandler $errorHandler,
+        bool $displayErrorDetails
+    ) {
+        $this->request = $request;
+        $this->errorHandler = $errorHandler;
+        $this->displayErrorDetails = $displayErrorDetails;
+    }
+
+    public function __invoke()
+    {
+        $error = error_get_last();
+        if ($error) {
+            $errorFile = $error['file'];
+            $errorLine = $error['line'];
+            $errorMessage = $error['message'];
+            $errorType = $error['type'];
+            $message = 'An error while processing your request. Please try again later.';
+
+            if ($this->displayErrorDetails) {
+                switch ($errorType) {
+                    case E_USER_ERROR:
+                        $message = "FATAL ERROR: {$errorMessage}. ";
+                        $message .= " on line {$errorLine} in file {$errorFile}.";
+                        break;
+
+                    case E_USER_WARNING:
+                        $message = "WARNING: {$errorMessage}";
+                        break;
+
+                    case E_USER_NOTICE:
+                        $message = "NOTICE: {$errorMessage}";
+                        break;
+
+                    default:
+                        $message = "ERROR: {$errorMessage}";
+                        $message .= " on line {$errorLine} in file {$errorFile}.";
+                        break;
+                }
+            }
+
+            $exception = new HttpInternalServerErrorException($this->request, $message);
+            $response = $this->errorHandler->__invoke($this->request, $exception, $this->displayErrorDetails, false, false);
+
+            $responseEmitter = new ResponseEmitter();
+            $responseEmitter->emit($response);
+        }
+    }
+}
diff --git a/api/src/Application/Middleware/SessionMiddleware.php b/api/src/Application/Middleware/SessionMiddleware.php
new file mode 100644
index 0000000..e06d788
--- /dev/null
+++ b/api/src/Application/Middleware/SessionMiddleware.php
@@ -0,0 +1,25 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Application\Middleware;
+
+use Psr\Http\Message\ResponseInterface as Response;
+use Psr\Http\Message\ServerRequestInterface as Request;
+use Psr\Http\Server\MiddlewareInterface as Middleware;
+use Psr\Http\Server\RequestHandlerInterface as RequestHandler;
+
+class SessionMiddleware implements Middleware
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function process(Request $request, RequestHandler $handler): Response
+    {
+        if (isset($_SERVER['HTTP_AUTHORIZATION'])) {
+            session_start();
+            $request = $request->withAttribute('session', $_SESSION);
+        }
+
+        return $handler->handle($request);
+    }
+}
diff --git a/api/src/Application/ResponseEmitter/ResponseEmitter.php b/api/src/Application/ResponseEmitter/ResponseEmitter.php
new file mode 100644
index 0000000..eeee133
--- /dev/null
+++ b/api/src/Application/ResponseEmitter/ResponseEmitter.php
@@ -0,0 +1,34 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Application\ResponseEmitter;
+
+use Psr\Http\Message\ResponseInterface;
+use Slim\ResponseEmitter as SlimResponseEmitter;
+
+class ResponseEmitter extends SlimResponseEmitter
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function emit(ResponseInterface $response): void
+    {
+        // This variable should be set to the allowed host from which your API can be accessed with
+        $origin = isset($_SERVER['HTTP_ORIGIN']) ? $_SERVER['HTTP_ORIGIN'] : '';
+
+        $response = $response
+            ->withHeader('Access-Control-Allow-Credentials', 'true')
+            ->withHeader('Access-Control-Allow-Origin', $origin)
+            ->withHeader('Access-Control-Allow-Headers', 'X-Requested-With, Content-Type, Accept, Origin, Authorization')
+            ->withHeader('Access-Control-Allow-Methods', 'GET, POST, PUT, PATCH, DELETE, OPTIONS')
+            ->withHeader('Cache-Control', 'no-store, no-cache, must-revalidate, max-age=0')
+            ->withAddedHeader('Cache-Control', 'post-check=0, pre-check=0')
+            ->withHeader('Pragma', 'no-cache');
+
+        if (ob_get_contents()) {
+            ob_clean();
+        }
+
+        parent::emit($response);
+    }
+}
diff --git a/api/src/Application/Settings/Settings.php b/api/src/Application/Settings/Settings.php
new file mode 100644
index 0000000..ab2143f
--- /dev/null
+++ b/api/src/Application/Settings/Settings.php
@@ -0,0 +1,30 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Application\Settings;
+
+class Settings implements SettingsInterface
+{
+    /**
+     * @var array
+     */
+    private $settings;
+
+    /**
+     * Settings constructor.
+     * @param array $settings
+     */
+    public function __construct(array $settings)
+    {
+        $this->settings = $settings;
+    }
+
+    /**
+     * @param string $key
+     * @return mixed
+     */
+    public function get(string $key = '')
+    {
+        return (empty($key)) ? $this->settings : $this->settings[$key];
+    }
+}
\ No newline at end of file
diff --git a/api/src/Application/Settings/SettingsInterface.php b/api/src/Application/Settings/SettingsInterface.php
new file mode 100644
index 0000000..f830cda
--- /dev/null
+++ b/api/src/Application/Settings/SettingsInterface.php
@@ -0,0 +1,13 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Application\Settings;
+
+interface SettingsInterface
+{
+    /**
+     * @param string $key
+     * @return mixed
+     */
+    public function get(string $key = '');
+}
\ No newline at end of file
diff --git a/api/src/Domain/DomainException/DomainException.php b/api/src/Domain/DomainException/DomainException.php
new file mode 100644
index 0000000..0ce9c69
--- /dev/null
+++ b/api/src/Domain/DomainException/DomainException.php
@@ -0,0 +1,10 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Domain\DomainException;
+
+use Exception;
+
+abstract class DomainException extends Exception
+{
+}
diff --git a/api/src/Domain/DomainException/DomainRecordNotFoundException.php b/api/src/Domain/DomainException/DomainRecordNotFoundException.php
new file mode 100644
index 0000000..10d9ff5
--- /dev/null
+++ b/api/src/Domain/DomainException/DomainRecordNotFoundException.php
@@ -0,0 +1,8 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Domain\DomainException;
+
+class DomainRecordNotFoundException extends DomainException
+{
+}
diff --git a/api/src/Domain/User/User.php b/api/src/Domain/User/User.php
new file mode 100644
index 0000000..0afc5db
--- /dev/null
+++ b/api/src/Domain/User/User.php
@@ -0,0 +1,88 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Domain\User;
+
+use JsonSerializable;
+
+class User implements JsonSerializable
+{
+    /**
+     * @var int|null
+     */
+    private $id;
+
+    /**
+     * @var string
+     */
+    private $username;
+
+    /**
+     * @var string
+     */
+    private $firstName;
+
+    /**
+     * @var string
+     */
+    private $lastName;
+
+    /**
+     * @param int|null  $id
+     * @param string    $username
+     * @param string    $firstName
+     * @param string    $lastName
+     */
+    public function __construct(?int $id, string $username, string $firstName, string $lastName)
+    {
+        $this->id = $id;
+        $this->username = strtolower($username);
+        $this->firstName = ucfirst($firstName);
+        $this->lastName = ucfirst($lastName);
+    }
+
+    /**
+     * @return int|null
+     */
+    public function getId(): ?int
+    {
+        return $this->id;
+    }
+
+    /**
+     * @return string
+     */
+    public function getUsername(): string
+    {
+        return $this->username;
+    }
+
+    /**
+     * @return string
+     */
+    public function getFirstName(): string
+    {
+        return $this->firstName;
+    }
+
+    /**
+     * @return string
+     */
+    public function getLastName(): string
+    {
+        return $this->lastName;
+    }
+
+    /**
+     * @return array
+     */
+    public function jsonSerialize()
+    {
+        return [
+            'id' => $this->id,
+            'username' => $this->username,
+            'firstName' => $this->firstName,
+            'lastName' => $this->lastName,
+        ];
+    }
+}
diff --git a/api/src/Domain/User/UserNotFoundException.php b/api/src/Domain/User/UserNotFoundException.php
new file mode 100644
index 0000000..21e1561
--- /dev/null
+++ b/api/src/Domain/User/UserNotFoundException.php
@@ -0,0 +1,11 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Domain\User;
+
+use TorstenHettstedt\TimekeepingApi\Domain\DomainException\DomainRecordNotFoundException;
+
+class UserNotFoundException extends DomainRecordNotFoundException
+{
+    public $message = 'The user you requested does not exist.';
+}
diff --git a/api/src/Domain/User/UserRepository.php b/api/src/Domain/User/UserRepository.php
new file mode 100644
index 0000000..603dfdb
--- /dev/null
+++ b/api/src/Domain/User/UserRepository.php
@@ -0,0 +1,19 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Domain\User;
+
+interface UserRepository
+{
+    /**
+     * @return User[]
+     */
+    public function findAll(): array;
+
+    /**
+     * @param int $id
+     * @return User
+     * @throws UserNotFoundException
+     */
+    public function findUserOfId(int $id): User;
+}
diff --git a/api/src/Infrastructure/Persistence/User/InMemoryUserRepository.php b/api/src/Infrastructure/Persistence/User/InMemoryUserRepository.php
new file mode 100644
index 0000000..f8032ec
--- /dev/null
+++ b/api/src/Infrastructure/Persistence/User/InMemoryUserRepository.php
@@ -0,0 +1,52 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Infrastructure\Persistence\User;
+
+use TorstenHettstedt\TimekeepingApi\Domain\User\User;
+use TorstenHettstedt\TimekeepingApi\Domain\User\UserNotFoundException;
+use TorstenHettstedt\TimekeepingApi\Domain\User\UserRepository;
+
+class InMemoryUserRepository implements UserRepository
+{
+    /**
+     * @var User[]
+     */
+    private $users;
+
+    /**
+     * InMemoryUserRepository constructor.
+     *
+     * @param array|null $users
+     */
+    public function __construct(array $users = null)
+    {
+        $this->users = $users ?? [
+            1 => new User(1, 'bill.gates', 'Bill', 'Gates'),
+            2 => new User(2, 'steve.jobs', 'Steve', 'Jobs'),
+            3 => new User(3, 'mark.zuckerberg', 'Mark', 'Zuckerberg'),
+            4 => new User(4, 'evan.spiegel', 'Evan', 'Spiegel'),
+            5 => new User(5, 'jack.dorsey', 'Jack', 'Dorsey'),
+        ];
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function findAll(): array
+    {
+        return array_values($this->users);
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function findUserOfId(int $id): User
+    {
+        if (!isset($this->users[$id])) {
+            throw new UserNotFoundException();
+        }
+
+        return $this->users[$id];
+    }
+}
diff --git a/api/tests/unit/Application/Actions/ActionTest.php b/api/tests/unit/Application/Actions/ActionTest.php
new file mode 100644
index 0000000..feb9900
--- /dev/null
+++ b/api/tests/unit/Application/Actions/ActionTest.php
@@ -0,0 +1,78 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Tests\Unit\Application\Actions;
+
+use TorstenHettstedt\TimekeepingApi\Application\Actions\Action;
+use TorstenHettstedt\TimekeepingApi\Application\Actions\ActionPayload;
+use DateTimeImmutable;
+use Psr\Http\Message\ResponseInterface as Response;
+use Psr\Log\LoggerInterface;
+use TorstenHettstedt\TimekeepingApi\Tests\Unit\TestCase;
+
+class ActionTest extends TestCase
+{
+    public function testActionSetsHttpCodeInRespond()
+    {
+        $app = $this->getAppInstance();
+        $container = $app->getContainer();
+        $logger = $container->get(LoggerInterface::class);
+
+        $testAction = new class($logger) extends Action {
+            public function __construct(
+                LoggerInterface $loggerInterface
+            ) {
+                parent::__construct($loggerInterface);
+            }
+
+            public function action(): Response
+            {
+                return $this->respond(
+                    new ActionPayload(
+                        202,
+                        [
+                            'willBeDoneAt' => (new DateTimeImmutable())->format(DateTimeImmutable::ATOM)
+                        ]
+                    )
+                );
+            }
+        };
+
+        $app->get('/test-action-response-code', $testAction);
+        $request = $this->createRequest('GET', '/test-action-response-code');
+        $response = $app->handle($request);
+
+        $this->assertEquals(202, $response->getStatusCode());
+    }
+
+    public function testActionSetsHttpCodeRespondData()
+    {
+        $app = $this->getAppInstance();
+        $container = $app->getContainer();
+        $logger = $container->get(LoggerInterface::class);
+
+        $testAction = new class($logger) extends Action {
+            public function __construct(
+                LoggerInterface $loggerInterface
+            ) {
+                parent::__construct($loggerInterface);
+            }
+
+            public function action(): Response
+            {
+                return $this->respondWithData(
+                    [
+                        'willBeDoneAt' => (new DateTimeImmutable())->format(DateTimeImmutable::ATOM)
+                    ],
+                    202
+                );
+            }
+        };
+
+        $app->get('/test-action-response-code', $testAction);
+        $request = $this->createRequest('GET', '/test-action-response-code');
+        $response = $app->handle($request);
+
+        $this->assertEquals(202, $response->getStatusCode());
+    }
+}
diff --git a/api/tests/unit/Application/Actions/User/ListUserActionTest.php b/api/tests/unit/Application/Actions/User/ListUserActionTest.php
new file mode 100644
index 0000000..31f154f
--- /dev/null
+++ b/api/tests/unit/Application/Actions/User/ListUserActionTest.php
@@ -0,0 +1,40 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Tests\Unit\Application\Actions\User;
+
+use TorstenHettstedt\TimekeepingApi\Application\Actions\ActionPayload;
+use TorstenHettstedt\TimekeepingApi\Domain\User\UserRepository;
+use TorstenHettstedt\TimekeepingApi\Domain\User\User;
+use DI\Container;
+use TorstenHettstedt\TimekeepingApi\Tests\Unit\TestCase;
+
+class ListUserActionTest extends TestCase
+{
+    public function testAction()
+    {
+        $app = $this->getAppInstance();
+
+        /** @var Container $container */
+        $container = $app->getContainer();
+
+        $user = new User(1, 'bill.gates', 'Bill', 'Gates');
+
+        $userRepositoryProphecy = $this->prophesize(UserRepository::class);
+        $userRepositoryProphecy
+            ->findAll()
+            ->willReturn([$user])
+            ->shouldBeCalledOnce();
+
+        $container->set(UserRepository::class, $userRepositoryProphecy->reveal());
+
+        $request = $this->createRequest('GET', '/users');
+        $response = $app->handle($request);
+
+        $payload = (string) $response->getBody();
+        $expectedPayload = new ActionPayload(200, [$user]);
+        $serializedPayload = json_encode($expectedPayload, JSON_PRETTY_PRINT);
+
+        $this->assertEquals($serializedPayload, $payload);
+    }
+}
diff --git a/api/tests/unit/Application/Actions/User/ViewUserActionTest.php b/api/tests/unit/Application/Actions/User/ViewUserActionTest.php
new file mode 100644
index 0000000..c9334bf
--- /dev/null
+++ b/api/tests/unit/Application/Actions/User/ViewUserActionTest.php
@@ -0,0 +1,79 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Tests\Unit\Application\Actions\User;
+
+use TorstenHettstedt\TimekeepingApi\Application\Actions\ActionError;
+use TorstenHettstedt\TimekeepingApi\Application\Actions\ActionPayload;
+use TorstenHettstedt\TimekeepingApi\Application\Handlers\HttpErrorHandler;
+use TorstenHettstedt\TimekeepingApi\Domain\User\User;
+use TorstenHettstedt\TimekeepingApi\Domain\User\UserNotFoundException;
+use TorstenHettstedt\TimekeepingApi\Domain\User\UserRepository;
+use DI\Container;
+use Slim\Middleware\ErrorMiddleware;
+use TorstenHettstedt\TimekeepingApi\Tests\Unit\TestCase;
+
+class ViewUserActionTest extends TestCase
+{
+    public function testAction()
+    {
+        $app = $this->getAppInstance();
+
+        /** @var Container $container */
+        $container = $app->getContainer();
+
+        $user = new User(1, 'bill.gates', 'Bill', 'Gates');
+
+        $userRepositoryProphecy = $this->prophesize(UserRepository::class);
+        $userRepositoryProphecy
+            ->findUserOfId(1)
+            ->willReturn($user)
+            ->shouldBeCalledOnce();
+
+        $container->set(UserRepository::class, $userRepositoryProphecy->reveal());
+
+        $request = $this->createRequest('GET', '/users/1');
+        $response = $app->handle($request);
+
+        $payload = (string) $response->getBody();
+        $expectedPayload = new ActionPayload(200, $user);
+        $serializedPayload = json_encode($expectedPayload, JSON_PRETTY_PRINT);
+
+        $this->assertEquals($serializedPayload, $payload);
+    }
+
+    public function testActionThrowsUserNotFoundException()
+    {
+        $app = $this->getAppInstance();
+
+        $callableResolver = $app->getCallableResolver();
+        $responseFactory = $app->getResponseFactory();
+
+        $errorHandler = new HttpErrorHandler($callableResolver, $responseFactory);
+        $errorMiddleware = new ErrorMiddleware($callableResolver, $responseFactory, true, false ,false);
+        $errorMiddleware->setDefaultErrorHandler($errorHandler);
+
+        $app->add($errorMiddleware);
+
+        /** @var Container $container */
+        $container = $app->getContainer();
+
+        $userRepositoryProphecy = $this->prophesize(UserRepository::class);
+        $userRepositoryProphecy
+            ->findUserOfId(1)
+            ->willThrow(new UserNotFoundException())
+            ->shouldBeCalledOnce();
+
+        $container->set(UserRepository::class, $userRepositoryProphecy->reveal());
+
+        $request = $this->createRequest('GET', '/users/1');
+        $response = $app->handle($request);
+
+        $payload = (string) $response->getBody();
+        $expectedError = new ActionError(ActionError::RESOURCE_NOT_FOUND, 'The user you requested does not exist.');
+        $expectedPayload = new ActionPayload(404, null, $expectedError);
+        $serializedPayload = json_encode($expectedPayload, JSON_PRETTY_PRINT);
+
+        $this->assertEquals($serializedPayload, $payload);
+    }
+}
diff --git a/api/tests/unit/Domain/User/UserTest.php b/api/tests/unit/Domain/User/UserTest.php
new file mode 100644
index 0000000..56bc1f2
--- /dev/null
+++ b/api/tests/unit/Domain/User/UserTest.php
@@ -0,0 +1,59 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Tests\Unit\Domain\User;
+
+use TorstenHettstedt\TimekeepingApi\Domain\User\User;
+use TorstenHettstedt\TimekeepingApi\Tests\Unit\TestCase;
+
+class UserTest extends TestCase
+{
+    public function userProvider()
+    {
+        return [
+            [1, 'bill.gates', 'Bill', 'Gates'],
+            [2, 'steve.jobs', 'Steve', 'Jobs'],
+            [3, 'mark.zuckerberg', 'Mark', 'Zuckerberg'],
+            [4, 'evan.spiegel', 'Evan', 'Spiegel'],
+            [5, 'jack.dorsey', 'Jack', 'Dorsey'],
+        ];
+    }
+
+    /**
+     * @dataProvider userProvider
+     * @param int    $id
+     * @param string $username
+     * @param string $firstName
+     * @param string $lastName
+     */
+    public function testGetters(int $id, string $username, string $firstName, string $lastName)
+    {
+        $user = new User($id, $username, $firstName, $lastName);
+
+        $this->assertEquals($id, $user->getId());
+        $this->assertEquals($username, $user->getUsername());
+        $this->assertEquals($firstName, $user->getFirstName());
+        $this->assertEquals($lastName, $user->getLastName());
+    }
+
+    /**
+     * @dataProvider userProvider
+     * @param int    $id
+     * @param string $username
+     * @param string $firstName
+     * @param string $lastName
+     */
+    public function testJsonSerialize(int $id, string $username, string $firstName, string $lastName)
+    {
+        $user = new User($id, $username, $firstName, $lastName);
+
+        $expectedPayload = json_encode([
+            'id' => $id,
+            'username' => $username,
+            'firstName' => $firstName,
+            'lastName' => $lastName,
+        ]);
+
+        $this->assertEquals($expectedPayload, json_encode($user));
+    }
+}
diff --git a/api/tests/unit/Infrastructure/Persistence/User/InMemoryUserRepositoryTest.php b/api/tests/unit/Infrastructure/Persistence/User/InMemoryUserRepositoryTest.php
new file mode 100644
index 0000000..975d1c4
--- /dev/null
+++ b/api/tests/unit/Infrastructure/Persistence/User/InMemoryUserRepositoryTest.php
@@ -0,0 +1,52 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Tests\Unit\Infrastructure\Persistence\User;
+
+use TorstenHettstedt\TimekeepingApi\Domain\User\User;
+use TorstenHettstedt\TimekeepingApi\Domain\User\UserNotFoundException;
+use TorstenHettstedt\TimekeepingApi\Infrastructure\Persistence\User\InMemoryUserRepository;
+use TorstenHettstedt\TimekeepingApi\Tests\Unit\TestCase;
+
+class InMemoryUserRepositoryTest extends TestCase
+{
+    public function testFindAll()
+    {
+        $user = new User(1, 'bill.gates', 'Bill', 'Gates');
+
+        $userRepository = new InMemoryUserRepository([1 => $user]);
+
+        $this->assertEquals([$user], $userRepository->findAll());
+    }
+
+    public function testFindAllUsersByDefault()
+    {
+        $users = [
+            1 => new User(1, 'bill.gates', 'Bill', 'Gates'),
+            2 => new User(2, 'steve.jobs', 'Steve', 'Jobs'),
+            3 => new User(3, 'mark.zuckerberg', 'Mark', 'Zuckerberg'),
+            4 => new User(4, 'evan.spiegel', 'Evan', 'Spiegel'),
+            5 => new User(5, 'jack.dorsey', 'Jack', 'Dorsey'),
+        ];
+
+        $userRepository = new InMemoryUserRepository();
+
+        $this->assertEquals(array_values($users), $userRepository->findAll());
+    }
+
+    public function testFindUserOfId()
+    {
+        $user = new User(1, 'bill.gates', 'Bill', 'Gates');
+
+        $userRepository = new InMemoryUserRepository([1 => $user]);
+
+        $this->assertEquals($user, $userRepository->findUserOfId(1));
+    }
+
+    public function testFindUserOfIdThrowsNotFoundException()
+    {
+        $userRepository = new InMemoryUserRepository([]);
+        $this->expectException(UserNotFoundException::class);
+        $userRepository->findUserOfId(1);
+    }
+}
diff --git a/api/tests/unit/TestCase.php b/api/tests/unit/TestCase.php
new file mode 100644
index 0000000..f789e27
--- /dev/null
+++ b/api/tests/unit/TestCase.php
@@ -0,0 +1,86 @@
+<?php
+declare(strict_types=1);
+
+namespace TorstenHettstedt\TimekeepingApi\Tests\Unit;
+
+use DI\ContainerBuilder;
+use Exception;
+use PHPUnit\Framework\TestCase as PHPUnit_TestCase;
+use Psr\Http\Message\ServerRequestInterface as Request;
+use Slim\App;
+use Slim\Factory\AppFactory;
+use Slim\Psr7\Factory\StreamFactory;
+use Slim\Psr7\Headers;
+use Slim\Psr7\Request as SlimRequest;
+use Slim\Psr7\Uri;
+
+class TestCase extends PHPUnit_TestCase
+{
+    /**
+     * @return App
+     * @throws Exception
+     */
+    protected function getAppInstance(): App
+    {
+        // Instantiate PHP-DI ContainerBuilder
+        $containerBuilder = new ContainerBuilder();
+
+        // Container intentionally not compiled for tests.
+
+        // Set up settings
+        $settings = require __DIR__ . '/../../app/settings.php';
+        $settings($containerBuilder);
+
+        // Set up dependencies
+        $dependencies = require __DIR__ . '/../../app/dependencies.php';
+        $dependencies($containerBuilder);
+
+        // Set up repositories
+        $repositories = require __DIR__ . '/../../app/repositories.php';
+        $repositories($containerBuilder);
+
+        // Build PHP-DI Container instance
+        $container = $containerBuilder->build();
+
+        // Instantiate the app
+        AppFactory::setContainer($container);
+        $app = AppFactory::create();
+
+        // Register middleware
+        $middleware = require __DIR__ . '/../../app/middleware.php';
+        $middleware($app);
+
+        // Register routes
+        $routes = require __DIR__ . '/../../app/routes.php';
+        $routes($app);
+
+        return $app;
+    }
+
+    /**
+     * @param string $method
+     * @param string $path
+     * @param array  $headers
+     * @param array  $cookies
+     * @param array  $serverParams
+     * @return Request
+     */
+    protected function createRequest(
+        string $method,
+        string $path,
+        array $headers = ['HTTP_ACCEPT' => 'application/json'],
+        array $cookies = [],
+        array $serverParams = []
+    ): Request {
+        $uri = new Uri('', '', 80, $path);
+        $handle = fopen('php://temp', 'w+');
+        $stream = (new StreamFactory())->createStreamFromResource($handle);
+
+        $h = new Headers();
+        foreach ($headers as $name => $value) {
+            $h->addHeader($name, $value);
+        }
+
+        return new SlimRequest($method, $uri, $h, $cookies, $serverParams, $stream);
+    }
+}
diff --git a/api/var/cache/.gitignore b/api/var/cache/.gitignore
new file mode 100644
index 0000000..d6b7ef3
--- /dev/null
+++ b/api/var/cache/.gitignore
@@ -0,0 +1,2 @@
+*
+!.gitignore
diff --git a/compose.yml b/compose.yml
new file mode 100644
index 0000000..9ab158a
--- /dev/null
+++ b/compose.yml
@@ -0,0 +1,14 @@
+volumes:
+    logs:
+        driver: local
+
+services:
+    api:
+        build: ./api/
+        environment:
+            docker: "true"
+        ports:
+            - 8090:80
+        volumes:
+            - ./api:/var/www
+            - logs:/var/www/logs