diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
new file mode 100644
index 0000000..3a37ec0
--- /dev/null
+++ b/.github/workflows/tests.yml
@@ -0,0 +1,39 @@
+name: tests
+
+on:
+  pull_request:
+  push:
+    branches:
+      - master
+      - '*.x'
+
+jobs:
+  tests:
+    runs-on: ubuntu-22.04
+
+    strategy:
+      matrix:
+        php: [7.1, 7.2, 7.3, 7.4, 8.0, 8.1, 8.2, 8.3]
+
+    name: PHP ${{ matrix.php }}
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+          tools: composer:v2
+          coverage: none
+
+      - name: Install composer dependencies
+        uses: nick-invision/retry@v3
+        with:
+          timeout_minutes: 5
+          max_attempts: 5
+          command: composer install --no-interaction --no-progress
+
+      - name: Execute tests
+        run: vendor/bin/phpunit
diff --git a/.gitignore b/.gitignore
index 4dfacb8..6d78f51 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,4 +2,5 @@
 /vendor
 
 # Files
+.phpunit.result.cache
 composer.lock
diff --git a/composer.json b/composer.json
index d916294..4f12f3b 100644
--- a/composer.json
+++ b/composer.json
@@ -19,7 +19,7 @@
         "guzzlehttp/guzzle": "^6.4|^7.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^8.0"
+        "phpunit/phpunit": "^7.0|^8.0"
     },
     "autoload": {
         "psr-4": {
diff --git a/phpunit.xml b/phpunit.xml
new file mode 100644
index 0000000..482b8c3
--- /dev/null
+++ b/phpunit.xml
@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="./vendor/phpunit/phpunit/phpunit.xsd"
+         bootstrap="vendor/autoload.php"
+         colors="true">
+    <testsuites>
+        <testsuite name="Unit">
+            <directory suffix="Test.php">./tests/Unit</directory>
+        </testsuite>
+    </testsuites>
+    <php>
+    </php>
+</phpunit>
diff --git a/tests/Unit/Api/Response/BaselinkerExceptionTest.php b/tests/Unit/Api/Response/BaselinkerExceptionTest.php
new file mode 100644
index 0000000..32a65e1
--- /dev/null
+++ b/tests/Unit/Api/Response/BaselinkerExceptionTest.php
@@ -0,0 +1,17 @@
+<?php
+
+namespace Api\Response;
+
+use Baselinker\Api\Response\BaselinkerException;
+use PHPUnit\Framework\TestCase;
+
+class BaselinkerExceptionTest extends TestCase
+{
+    public function testGetters(): void
+    {
+        $exception = new BaselinkerException('Message', 'CODE');
+
+        $this->assertEquals('Message', $exception->responseMessage());
+        $this->assertEquals('CODE', $exception->responseCode());
+    }
+}
diff --git a/tests/Unit/Api/Response/ResponseTest.php b/tests/Unit/Api/Response/ResponseTest.php
new file mode 100644
index 0000000..6fee803
--- /dev/null
+++ b/tests/Unit/Api/Response/ResponseTest.php
@@ -0,0 +1,54 @@
+<?php
+
+namespace Api\Response;
+
+use Baselinker\Api\Response\BaselinkerException;
+use Baselinker\Api\Response\Response;
+use GuzzleHttp\Psr7\Response as GuzzleResponse;
+use PHPUnit\Framework\TestCase;
+
+class ResponseTest extends TestCase
+{
+    public function testGetters(): void
+    {
+        $body = json_encode([
+            'status' => 'SUCCESS',
+            'list' => [
+                'foo' => 'bar',
+            ],
+        ]);
+
+        $response = new Response(
+            new GuzzleResponse(200, [], $body)
+        );
+
+        $this->assertEquals('{"status":"SUCCESS","list":{"foo":"bar"}}', $response->contents());
+
+        $this->assertEquals([
+            'status' => 'SUCCESS',
+            'list' => [
+                'foo' => 'bar',
+            ],
+        ], $response->toArray());
+
+        $this->assertEquals([
+            'foo' => 'bar',
+        ], $response->getParameter('list'));
+    }
+
+    public function testWithErrorThrowsException(): void
+    {
+        $body = json_encode([
+            'status' => 'ERROR',
+            'error_code' => 'CODE',
+            'error_message' => 'Message',
+        ]);
+
+        $this->expectException(BaselinkerException::class);
+        $this->expectExceptionMessage('[CODE] Message');
+
+        new Response(
+            new GuzzleResponse(200, [], $body)
+        );
+    }
+}
diff --git a/tests/Unit/BaselinkerTest.php b/tests/Unit/BaselinkerTest.php
new file mode 100644
index 0000000..aac35cd
--- /dev/null
+++ b/tests/Unit/BaselinkerTest.php
@@ -0,0 +1,69 @@
+<?php
+
+namespace Baselinker\Tests\Unit;
+
+use Baselinker\Api\Request\CourierShipmentsInterface;
+use Baselinker\Api\Request\ExternalStoragesInterface;
+use Baselinker\Api\Request\OrderReturnsInterface;
+use Baselinker\Api\Request\OrdersInterface;
+use Baselinker\Api\Request\ProductCatalogInterface;
+use Baselinker\Baselinker;
+use PHPUnit\Framework\TestCase;
+
+class BaselinkerTest extends TestCase
+{
+    public function testProductCatalog(): void
+    {
+        $baselinker = new Baselinker([
+            'token' => 'foo',
+        ]);
+
+        $productCatalog = $baselinker->productCatalog();
+
+        $this->assertInstanceOf(ProductCatalogInterface::class, $productCatalog);
+    }
+
+    public function testExternalStorages(): void
+    {
+        $baselinker = new Baselinker([
+            'token' => 'foo',
+        ]);
+
+        $externalStorages = $baselinker->externalStorages();
+
+        $this->assertInstanceOf(ExternalStoragesInterface::class, $externalStorages);
+    }
+
+    public function testOrders(): void
+    {
+        $baselinker = new Baselinker([
+            'token' => 'foo',
+        ]);
+
+        $orders = $baselinker->orders();
+
+        $this->assertInstanceOf(OrdersInterface::class, $orders);
+    }
+
+    public function testOrderReturns(): void
+    {
+        $baselinker = new Baselinker([
+            'token' => 'foo',
+        ]);
+
+        $orderReturns = $baselinker->orderReturns();
+
+        $this->assertInstanceOf(OrderReturnsInterface::class, $orderReturns);
+    }
+
+    public function testCourierShipments(): void
+    {
+        $baselinker = new Baselinker([
+            'token' => 'foo',
+        ]);
+
+        $courierShipments = $baselinker->courierShipments();
+
+        $this->assertInstanceOf(CourierShipmentsInterface::class, $courierShipments);
+    }
+}
diff --git a/tests/Unit/ConfigTest.php b/tests/Unit/ConfigTest.php
new file mode 100644
index 0000000..be96033
--- /dev/null
+++ b/tests/Unit/ConfigTest.php
@@ -0,0 +1,50 @@
+<?php
+
+namespace Baselinker\Tests\Unit;
+
+use Baselinker\Config;
+use InvalidArgumentException;
+use Iterator;
+use PHPUnit\Framework\TestCase;
+
+class ConfigTest extends TestCase
+{
+    public function testGetToken(): void
+    {
+        $config = new Config([
+            'token' => 'foo',
+        ]);
+
+        $this->assertEquals('foo', $config->getToken());
+    }
+
+    /**
+     * @dataProvider invalidConfigDataProvider
+     */
+    public function testSetWithInvalidTokenThrowsException(array $config): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('Parameter "token" must be provided in the configuration.');
+
+        new Config($config);
+    }
+
+    public static function invalidConfigDataProvider(): Iterator
+    {
+        yield 'empty config' => [
+            [],
+        ];
+
+        yield 'null as token' => [
+            [
+                'token' => null,
+            ]
+        ];
+
+        yield 'empty string as token' => [
+            [
+                'token' => '',
+            ]
+        ];
+    }
+}