diff --git a/src/Attachment/AttachmentsCollection.php b/src/Attachment/AttachmentsCollection.php
index a7422a6..8da9159 100644
--- a/src/Attachment/AttachmentsCollection.php
+++ b/src/Attachment/AttachmentsCollection.php
@@ -5,6 +5,7 @@
 use Countable;
 use IteratorAggregate;
 use Soap\Psr18AttachmentsMiddleware\Exception\AttachmentNotFoundException;
+use Traversable;
 
 /**
  * @template-implements \IteratorAggregate<int, Attachment>
@@ -24,7 +25,7 @@ public function __construct(Attachment ... $attachments)
         $this->attachments = $attachments;
     }
 
-    public function getIterator(): iterable
+    public function getIterator(): Traversable
     {
         yield from $this->attachments;
     }
diff --git a/tests/Unit/Attachment/AttachmentsCollectionTest.php b/tests/Unit/Attachment/AttachmentsCollectionTest.php
new file mode 100644
index 0000000..9770ea6
--- /dev/null
+++ b/tests/Unit/Attachment/AttachmentsCollectionTest.php
@@ -0,0 +1,57 @@
+<?php declare(strict_types=1);
+
+namespace SoapTest\Psr18AttachmentsMiddleware\Unit\Attachment;
+
+use Phpro\ResourceStream\Factory\MemoryStream;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+use Soap\Psr18AttachmentsMiddleware\Attachment\Attachment;
+use Soap\Psr18AttachmentsMiddleware\Attachment\AttachmentsCollection;
+use Soap\Psr18AttachmentsMiddleware\Exception\AttachmentNotFoundException;
+
+final class AttachmentsCollectionTest extends TestCase
+{
+    #[Test]
+    public function it_can_contain_attachments(): void
+    {
+        $collection = new AttachmentsCollection(
+            $attachment1 = Attachment::create('filename.pdf', MemoryStream::create()),
+            $attachment2 = Attachment::create('filename.jpg', MemoryStream::create()),
+        );
+
+        static::assertCount(2, $collection);
+        static::assertSame([$attachment1, $attachment2], [...$collection]);
+    }
+
+    #[Test]
+    public function it_can_add_an_item_mutably(): void
+    {
+        $collection = new AttachmentsCollection();
+        $collection->add($attachment = Attachment::create('filename.pdf', MemoryStream::create()));
+
+        static::assertCount(1, $collection);
+        static::assertSame([$attachment], [...$collection]);
+    }
+
+    #[Test]
+    public function it_can_find_an_attachment_by_id(): void
+    {
+        $collection = new AttachmentsCollection(
+            $attachment1 = Attachment::create('filename.pdf', MemoryStream::create()),
+            $attachment2 = Attachment::create('filename.jpg', MemoryStream::create()),
+        );
+
+        static::assertSame($attachment1, $collection->findById($attachment1->id));
+    }
+
+    #[Test]
+    public function it_can_fail_finding_an_attachment_by_id(): void
+    {
+        $collection = new AttachmentsCollection(
+            Attachment::create('filename.pdf', MemoryStream::create()),
+        );
+
+        $this->expectExceptionObject(AttachmentNotFoundException::withId('not-found'));
+        $collection->findById('not-found');
+    }
+}
diff --git a/tests/Unit/Attachment/IdGeneratorTest.php b/tests/Unit/Attachment/IdGeneratorTest.php
new file mode 100644
index 0000000..0c3cc27
--- /dev/null
+++ b/tests/Unit/Attachment/IdGeneratorTest.php
@@ -0,0 +1,21 @@
+<?php declare(strict_types=1);
+
+namespace SoapTest\Psr18AttachmentsMiddleware\Unit\Attachment;
+
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+use Soap\Psr18AttachmentsMiddleware\Attachment\IdGenerator;
+
+final class IdGeneratorTest extends TestCase
+{
+    #[Test]
+    public function it_can_generate_a_random_id(): void
+    {
+        $id1 = IdGenerator::generate();
+        $id2 = IdGenerator::generate();
+
+        static::assertNotSame($id1, $id2);
+        static::assertSame(16, mb_strlen($id1));
+        static::assertSame(16, mb_strlen($id2));
+    }
+}
diff --git a/tests/Unit/Encoding/Xop/XopIncludeEncoderTest.php b/tests/Unit/Encoding/Xop/XopIncludeEncoderTest.php
new file mode 100644
index 0000000..3916f7c
--- /dev/null
+++ b/tests/Unit/Encoding/Xop/XopIncludeEncoderTest.php
@@ -0,0 +1,73 @@
+<?php declare(strict_types=1);
+
+namespace SoapTest\Psr18AttachmentsMiddleware\Unit\Encoding\Xop;
+
+use Phpro\ResourceStream\Factory\MemoryStream;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+use Soap\Encoding\Encoder\Context;
+use Soap\Encoding\EncoderRegistry;
+use Soap\Engine\Metadata\Collection\MethodCollection;
+use Soap\Engine\Metadata\Collection\TypeCollection;
+use Soap\Engine\Metadata\InMemoryMetadata;
+use Soap\Engine\Metadata\Model\XsdType;
+use Soap\Psr18AttachmentsMiddleware\Attachment\Attachment;
+use Soap\Psr18AttachmentsMiddleware\Encoding\Xop\XopIncludeEncoder;
+use Soap\Psr18AttachmentsMiddleware\Storage\AttachmentStorage;
+use Soap\Psr18AttachmentsMiddleware\Storage\AttachmentStorageInterface;
+use Soap\WsdlReader\Model\Definitions\Namespaces;
+
+final class XopIncludeEncoderTest extends TestCase
+{
+    #[Test]
+    public function it_can_encode_xop_include_attachment(): void
+    {
+        $encoder = $this->createEncoder(
+            $storage = $this->createStorage()
+        );
+        $iso = $encoder->iso($this->createContext());
+
+        $result = $iso->to(
+            $attachment = new Attachment('foo', 'file.pdf', 'application/pdf', MemoryStream::create())
+        );
+
+        static::assertSame('<xop:Include href="cid:foo" xmlns:xop="http://www.w3.org/2004/08/xop/include"/>', $result);
+        static::assertSame($attachment, $storage->requestAttachments()->findById('foo'));
+    }
+
+    #[Test]
+    public function it_can_decode_xop_include_attachment(): void
+    {
+        $encoder = $this->createEncoder(
+            $storage = $this->createStorage()
+        );
+        $iso = $encoder->iso($this->createContext());
+
+        $storage->responseAttachments()->add(
+            $attachment = new Attachment('foo', 'file.pdf', 'application/pdf', MemoryStream::create())
+        );
+        $result = $iso->from('<xop:Include href="cid:foo" xmlns:xop="http://www.w3.org/2004/08/xop/include"/>');
+
+        static::assertSame($attachment, $result);
+    }
+
+    private function createStorage(): AttachmentStorageInterface
+    {
+        return new AttachmentStorage();
+    }
+
+    private function createEncoder(AttachmentStorageInterface $storage): XopIncludeEncoder
+    {
+        return new XopIncludeEncoder($storage);
+    }
+
+    private function createContext(): Context
+    {
+        return new Context(
+            XsdType::any(),
+            new InMemoryMetadata(new TypeCollection(), new MethodCollection()),
+            EncoderRegistry::default(),
+            new Namespaces([], []),
+        );
+    }
+}
diff --git a/tests/Unit/Exception/AttachmentNotFoundExceptionTest.php b/tests/Unit/Exception/AttachmentNotFoundExceptionTest.php
new file mode 100644
index 0000000..b91bd03
--- /dev/null
+++ b/tests/Unit/Exception/AttachmentNotFoundExceptionTest.php
@@ -0,0 +1,22 @@
+<?php declare(strict_types=1);
+
+namespace SoapTest\Psr18AttachmentsMiddleware\Unit\Exception;
+
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+use Soap\Engine\Exception\RuntimeException;
+use Soap\Psr18AttachmentsMiddleware\Exception\AttachmentNotFoundException;
+
+final class AttachmentNotFoundExceptionTest extends TestCase
+{
+    #[Test]
+    public function it_can_throw_by_id(): void
+    {
+        $exception = AttachmentNotFoundException::withId('foo');
+
+        $this->expectException(AttachmentNotFoundException::class);
+        $this->expectException(RuntimeException::class);
+        $this->expectExceptionMessage('Attachment with id "foo" can not be found.');
+        throw $exception;
+    }
+}
diff --git a/tests/Unit/Exception/SoapMessageNotFoundExceptionTest.php b/tests/Unit/Exception/SoapMessageNotFoundExceptionTest.php
new file mode 100644
index 0000000..598ad15
--- /dev/null
+++ b/tests/Unit/Exception/SoapMessageNotFoundExceptionTest.php
@@ -0,0 +1,23 @@
+<?php declare(strict_types=1);
+
+namespace SoapTest\Psr18AttachmentsMiddleware\Unit\Exception;
+
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+use Soap\Engine\Exception\RuntimeException;
+use Soap\Psr18AttachmentsMiddleware\Exception\SoapMessageNotFoundException;
+
+final class SoapMessageNotFoundExceptionTest extends TestCase
+{
+    #[Test]
+    public function it_can_throw_from_multipart_context(): void
+    {
+        $exception = SoapMessageNotFoundException::insideMultipart('soapmessage', 'application/soap+xml');
+
+        $this->expectException(SoapMessageNotFoundException::class);
+        $this->expectException(RuntimeException::class);
+        $this->expectExceptionMessage('Soap message with id "soapmessage" and type "application/soap+xml" can not be found inside multipart response.');
+        throw $exception;
+    }
+
+}
diff --git a/tests/Unit/Middleware/AttachmentsMiddlewareTest.php b/tests/Unit/Middleware/AttachmentsMiddlewareTest.php
new file mode 100644
index 0000000..1550d19
--- /dev/null
+++ b/tests/Unit/Middleware/AttachmentsMiddlewareTest.php
@@ -0,0 +1,10 @@
+<?php declare(strict_types=1);
+
+namespace SoapTest\Psr18AttachmentsMiddleware\Unit\Middleware;
+
+use PHPUnit\Framework\TestCase;
+
+final class AttachmentsMiddlewareTest extends TestCase
+{
+
+}
diff --git a/tests/Unit/Multipart/RequestBuilderTest.php b/tests/Unit/Multipart/RequestBuilderTest.php
new file mode 100644
index 0000000..4ac5879
--- /dev/null
+++ b/tests/Unit/Multipart/RequestBuilderTest.php
@@ -0,0 +1,10 @@
+<?php declare(strict_types=1);
+
+namespace SoapTest\Psr18AttachmentsMiddleware\Unit\Multipart;
+
+use PHPUnit\Framework\TestCase;
+
+final class RequestBuilderTest extends TestCase
+{
+
+}
diff --git a/tests/Unit/Multipart/ResponseBuilderTest.php b/tests/Unit/Multipart/ResponseBuilderTest.php
new file mode 100644
index 0000000..d74c14d
--- /dev/null
+++ b/tests/Unit/Multipart/ResponseBuilderTest.php
@@ -0,0 +1,10 @@
+<?php declare(strict_types=1);
+
+namespace SoapTest\Psr18AttachmentsMiddleware\Unit\Multipart;
+
+use PHPUnit\Framework\TestCase;
+
+final class ResponseBuilderTest extends TestCase
+{
+
+}
diff --git a/tests/Unit/Storage/AttachmentStorageTest.php b/tests/Unit/Storage/AttachmentStorageTest.php
new file mode 100644
index 0000000..40cf13c
--- /dev/null
+++ b/tests/Unit/Storage/AttachmentStorageTest.php
@@ -0,0 +1,62 @@
+<?php declare(strict_types=1);
+
+namespace SoapTest\Psr18AttachmentsMiddleware\Unit\Storage;
+
+use Phpro\ResourceStream\Factory\MemoryStream;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+use Soap\Psr18AttachmentsMiddleware\Attachment\Attachment;
+use Soap\Psr18AttachmentsMiddleware\Attachment\AttachmentsCollection;
+use Soap\Psr18AttachmentsMiddleware\Storage\AttachmentStorage;
+
+final class AttachmentStorageTest extends TestCase
+{
+    #[Test]
+    public function it_contains_request_attachments(): void
+    {
+        $attachments = $this->createStorage()->requestAttachments();
+
+        static::assertEquals(new AttachmentsCollection(), $attachments);
+    }
+
+    #[Test]
+    public function it_contains_response_attachments(): void
+    {
+        $attachments = $this->createStorage()->responseAttachments();
+
+        static::assertEquals(new AttachmentsCollection(), $attachments);
+    }
+
+    #[Test]
+    public function it_can_reset_request_attachments(): void
+    {
+        $storage = $this->createStorage();
+        $attachments = $storage->requestAttachments()->add(
+            Attachment::create('foo.png', MemoryStream::create())
+        );
+        $storage->resetRequestAttachments();
+        $newAttachments = $storage->requestAttachments();
+
+        static::assertNotSame($attachments, $newAttachments);
+        static::assertEquals(new AttachmentsCollection(), $storage->requestAttachments());
+    }
+
+    #[Test]
+    public function it_can_reset_response_attachments(): void
+    {
+        $storage = $this->createStorage();
+        $attachments = $storage->responseAttachments()->add(
+            Attachment::create('foo.png', MemoryStream::create())
+        );
+        $storage->resetResponseAttachments();
+        $newAttachments = $storage->responseAttachments();
+
+        static::assertNotSame($attachments, $newAttachments);
+        static::assertEquals(new AttachmentsCollection(), $storage->responseAttachments());
+    }
+
+    private function createStorage(): AttachmentStorage
+    {
+        return new AttachmentStorage();
+    }
+}