diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 3ca311b..2068a35 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -29,6 +29,27 @@ jobs:
       - name: Run friendsofphp/php-cs-fixer
         run: composer php-cs
 
+  type-checker:
+    name: Type Checker
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Install PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: 8.0
+          coverage: none
+
+      - name: Install composer dependencies
+        uses: ramsey/composer-install@v1
+
+      - name: Run vimeo/psalm
+        run: composer psalm
+
   tests:
     name: Tests
 
diff --git a/composer.json b/composer.json
index 7c87bd4..9be5435 100644
--- a/composer.json
+++ b/composer.json
@@ -20,7 +20,8 @@
         "php-http/mock-client": "^1.4",
         "phpunit/phpunit": "^8.5",
         "symfony/http-client": "^5.3",
-        "symfony/var-dumper": "^5.3"
+        "symfony/var-dumper": "^5.3",
+        "vimeo/psalm": "^4.8"
     },
     "config": {
         "sort-packages": true
@@ -41,6 +42,7 @@
     "minimum-stability": "dev",
     "prefer-stable": true,
     "scripts": {
+        "psalm": "@php vendor/bin/psalm --no-progress",
         "php-cs": "@php vendor/bin/php-cs-fixer fix --dry-run",
         "php-cs-fix": "@php vendor/bin/php-cs-fixer fix",
         "test": "@php vendor/bin/phpunit --configuration phpunit.dist.xml",
diff --git a/psalm.xml b/psalm.xml
new file mode 100644
index 0000000..f6529b3
--- /dev/null
+++ b/psalm.xml
@@ -0,0 +1,37 @@
+<?xml version="1.0"?>
+<psalm
+    errorLevel="5"
+    resolveFromConfigFile="true"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xmlns="https://getpsalm.org/schema/config"
+    xsi:schemaLocation="https://getpsalm.org/schema/config vendor/vimeo/psalm/config.xsd"
+>
+    <projectFiles>
+        <directory name="src" />
+        <ignoreFiles>
+            <directory name="vendor" />
+        </ignoreFiles>
+    </projectFiles>
+    <issueHandlers>
+        <MissingConstructor>
+            <errorLevel type="suppress">
+                <directory name="src/Common/Dto"/>
+                <directory name="src/Common/ValueObject"/>
+                <directory name="src/Confirmation/ValueObject"/>
+                <directory name="src/Itn/ValueObject"/>
+                <directory name="src/Itn/Dto"/>
+                <directory name="src/PaywayList/ValueObject"/>
+                <directory name="src/PaywayList/Dto"/>
+                <directory name="src/RegulationList/ValueObject"/>
+                <directory name="src/RegulationList/Dto"/>
+                <directory name="src/Transaction/ValueObject"/>
+                <directory name="src/Transaction/Dto"/>
+            </errorLevel>
+        </MissingConstructor>
+        <UndefinedMethod>
+            <errorLevel type="info">
+                <file name="src/Common/Builder/ServiceDtoBuilder.php"/>
+            </errorLevel>
+        </UndefinedMethod>
+    </issueHandlers>
+</psalm>
diff --git a/src/Client.php b/src/Client.php
index c977472..9f29fe0 100644
--- a/src/Client.php
+++ b/src/Client.php
@@ -4,12 +4,12 @@
 
 namespace BlueMedia;
 
+use BlueMedia\Common\ValueObject\HashableInterface;
 use BlueMedia\Confirmation\Builder\ConfirmationVOBuilder;
 use BlueMedia\HttpClient\HttpClientInterface;
 use BlueMedia\Hash\HashChecker;
 use BlueMedia\Itn\Builder\ItnVOBuilder;
 use BlueMedia\Itn\Decoder\ItnDecoder;
-use BlueMedia\Serializer\SerializableInterface;
 use BlueMedia\Transaction\View;
 use BlueMedia\Itn\ValueObject\Itn;
 use BlueMedia\HttpClient\HttpClient;
@@ -40,7 +40,7 @@ final class Client
     private $configuration;
 
     /**
-     * @var HttpClientInterface|null
+     * @var HttpClientInterface
      */
     private $httpClient;
 
@@ -126,11 +126,9 @@ public function doItnIn(string $itn): Response
     /**
      * Returns response for ITN IN request.
      *
-     * @return Response
-     *
      * @api
      */
-    public function doItnInResponse(Itn $itn, bool $transactionConfirmed = true)
+    public function doItnInResponse(Itn $itn, bool $transactionConfirmed = true): Response
     {
         return new Response(ItnResponseBuilder::build($itn, $transactionConfirmed, $this->configuration));
     }
@@ -196,7 +194,7 @@ public function getRegulationList(string $gatewayUrl): Response
      *
      * @api
      */
-    public function checkHash(SerializableInterface $data): bool
+    public function checkHash(HashableInterface $data): bool
     {
         return HashChecker::checkHash($data, $this->configuration);
     }
diff --git a/src/Common/Builder/ServiceDtoBuilder.php b/src/Common/Builder/ServiceDtoBuilder.php
index 32c02e0..c8da477 100644
--- a/src/Common/Builder/ServiceDtoBuilder.php
+++ b/src/Common/Builder/ServiceDtoBuilder.php
@@ -11,6 +11,12 @@
 
 final class ServiceDtoBuilder
 {
+    /**
+     * @template T of AbstractDto
+     * @psalm-param class-string<T> $type
+     *
+     * @return T
+     */
     public static function build(array $data, string $type, Configuration $configuration): AbstractDto
     {
         $serializer = new Serializer();
diff --git a/src/Common/Dto/AbstractDto.php b/src/Common/Dto/AbstractDto.php
index f860d0c..6aa1369 100644
--- a/src/Common/Dto/AbstractDto.php
+++ b/src/Common/Dto/AbstractDto.php
@@ -4,8 +4,8 @@
 
 namespace BlueMedia\Common\Dto;
 
+use BlueMedia\Common\ValueObject\AbstractValueObject;
 use BlueMedia\HttpClient\ValueObject\Request;
-use BlueMedia\Serializer\SerializableInterface;
 use JMS\Serializer\Annotation\Type;
 
 abstract class AbstractDto
@@ -39,5 +39,5 @@ public function getRequest(): ?Request
         return $this->request;
     }
 
-    abstract public function getRequestData(): SerializableInterface;
+    abstract public function getRequestData(): AbstractValueObject;
 }
diff --git a/src/Common/Parser/ServiceResponseParser.php b/src/Common/Parser/ServiceResponseParser.php
index e9535d9..9d014db 100644
--- a/src/Common/Parser/ServiceResponseParser.php
+++ b/src/Common/Parser/ServiceResponseParser.php
@@ -9,6 +9,12 @@
 
 final class ServiceResponseParser extends ResponseParser
 {
+    /**
+     * @template T
+     * @psalm-param class-string<T> $type
+     *
+     * @return T
+     */
     public function parseListResponse(string $type): SerializableInterface
     {
         $this->isErrorResponse();
diff --git a/src/Common/ValueObject/HashableInterface.php b/src/Common/ValueObject/HashableInterface.php
new file mode 100644
index 0000000..9cb5cd2
--- /dev/null
+++ b/src/Common/ValueObject/HashableInterface.php
@@ -0,0 +1,10 @@
+<?php
+
+namespace BlueMedia\Common\ValueObject;
+
+interface HashableInterface
+{
+    public function toArray(): array;
+
+    public function getHash(): string;
+}
diff --git a/src/Confirmation/ValueObject/Confirmation.php b/src/Confirmation/ValueObject/Confirmation.php
index 4fe3cdc..cf905c4 100644
--- a/src/Confirmation/ValueObject/Confirmation.php
+++ b/src/Confirmation/ValueObject/Confirmation.php
@@ -4,6 +4,7 @@
 
 namespace BlueMedia\Confirmation\ValueObject;
 
+use BlueMedia\Common\ValueObject\HashableInterface;
 use JMS\Serializer\Annotation\Type;
 use JMS\Serializer\Annotation\AccessorOrder;
 use BlueMedia\Serializer\SerializableInterface;
@@ -17,7 +18,7 @@
  *      "hash"
  * })
  */
-class Confirmation extends AbstractValueObject implements SerializableInterface
+class Confirmation extends AbstractValueObject implements SerializableInterface, HashableInterface
 {
     /**
      * @var string
diff --git a/src/Hash/HashChecker.php b/src/Hash/HashChecker.php
index 6b69aaa..8ec1b6d 100644
--- a/src/Hash/HashChecker.php
+++ b/src/Hash/HashChecker.php
@@ -4,12 +4,12 @@
 
 namespace BlueMedia\Hash;
 
+use BlueMedia\Common\ValueObject\HashableInterface;
 use BlueMedia\Configuration;
-use BlueMedia\Serializer\SerializableInterface;
 
 final class HashChecker
 {
-    public static function checkHash(SerializableInterface $data, Configuration $configuration): bool
+    public static function checkHash(HashableInterface $data, Configuration $configuration): bool
     {
         $dataHash = HashGenerator::generateHash(
             $data->toArray(),
diff --git a/src/Itn/Builder/ItnDtoBuilder.php b/src/Itn/Builder/ItnDtoBuilder.php
index 1699114..52bf57e 100644
--- a/src/Itn/Builder/ItnDtoBuilder.php
+++ b/src/Itn/Builder/ItnDtoBuilder.php
@@ -5,18 +5,18 @@
 namespace BlueMedia\Itn\Builder;
 
 use BlueMedia\Itn\Dto\ItnDto;
-use BlueMedia\Serializer\SerializableInterface;
 use BlueMedia\Serializer\Serializer;
 use BlueMedia\Common\Util\XMLParser;
 
 final class ItnDtoBuilder
 {
-    public static function build(string $itnData): SerializableInterface
+    public static function build(string $itnData): ItnDto
     {
         $serializer = new Serializer();
         $xmlData = XMLParser::parse($itnData);
 
         $xmlTransaction = $xmlData->transactions->transaction->asXML();
+        /** @var ItnDto $itnDto */
         $itnDto = $serializer->deserializeXml($xmlTransaction, ItnDto::class);
 
         $itnDto->getItn()->setServiceID((string) $xmlData->serviceID);
diff --git a/src/Itn/Dto/ItnDto.php b/src/Itn/Dto/ItnDto.php
index 8152655..96bbbe0 100644
--- a/src/Itn/Dto/ItnDto.php
+++ b/src/Itn/Dto/ItnDto.php
@@ -5,6 +5,7 @@
 namespace BlueMedia\Itn\Dto;
 
 use BlueMedia\Common\Dto\AbstractDto;
+use BlueMedia\Common\ValueObject\AbstractValueObject;
 use BlueMedia\Itn\ValueObject\Itn;
 use BlueMedia\Serializer\SerializableInterface;
 use JMS\Serializer\Annotation\XmlList;
@@ -24,7 +25,7 @@ public function getItn(): Itn
         return $this->itn;
     }
 
-    public function getRequestData(): SerializableInterface
+    public function getRequestData(): AbstractValueObject
     {
         return $this->getItn();
     }
diff --git a/src/PaywayList/Dto/PaywayListDto.php b/src/PaywayList/Dto/PaywayListDto.php
index 89c9147..4819251 100644
--- a/src/PaywayList/Dto/PaywayListDto.php
+++ b/src/PaywayList/Dto/PaywayListDto.php
@@ -5,8 +5,8 @@
 namespace BlueMedia\PaywayList\Dto;
 
 use BlueMedia\Common\Dto\AbstractDto;
+use BlueMedia\Common\ValueObject\AbstractValueObject;
 use JMS\Serializer\Annotation\Type;
-use BlueMedia\Serializer\SerializableInterface;
 use BlueMedia\PaywayList\ValueObject\PaywayList;
 
 final class PaywayListDto extends AbstractDto
@@ -22,7 +22,7 @@ public function getPaywayList(): PaywayList
         return $this->paywayList;
     }
 
-    public function getRequestData(): SerializableInterface
+    public function getRequestData(): AbstractValueObject
     {
         return $this->getPaywayList();
     }
diff --git a/src/RegulationList/Dto/RegulationListDto.php b/src/RegulationList/Dto/RegulationListDto.php
index b6e0986..013534f 100644
--- a/src/RegulationList/Dto/RegulationListDto.php
+++ b/src/RegulationList/Dto/RegulationListDto.php
@@ -4,9 +4,9 @@
 
 namespace BlueMedia\RegulationList\Dto;
 
+use BlueMedia\Common\ValueObject\AbstractValueObject;
 use JMS\Serializer\Annotation\Type;
 use BlueMedia\Common\Dto\AbstractDto;
-use BlueMedia\Serializer\SerializableInterface;
 use BlueMedia\RegulationList\ValueObject\RegulationList;
 
 final class RegulationListDto extends AbstractDto
@@ -22,7 +22,7 @@ public function getRegulationList(): RegulationList
         return $this->regulationList;
     }
 
-    public function getRequestData(): SerializableInterface
+    public function getRequestData(): AbstractValueObject
     {
         return $this->getRegulationList();
     }
diff --git a/src/Serializer/Serializer.php b/src/Serializer/Serializer.php
index d2d2d2b..e75ea40 100644
--- a/src/Serializer/Serializer.php
+++ b/src/Serializer/Serializer.php
@@ -4,7 +4,6 @@
 
 namespace BlueMedia\Serializer;
 
-use BlueMedia\Common\Dto\AbstractDto;
 use JMS\Serializer\SerializerBuilder;
 use JMS\Serializer\Naming\SerializedNameAnnotationStrategy;
 use JMS\Serializer\Naming\IdenticalPropertyNamingStrategy;
@@ -13,6 +12,9 @@ final class Serializer implements SerializerInterface
 {
     private const XML_TYPE = 'xml';
 
+    /**
+     * @var \JMS\Serializer\Serializer
+     */
     private $serializer;
 
     public function __construct()
@@ -26,7 +28,13 @@ public function __construct()
             ->build();
     }
 
-    public function serializeDataToDto(array $data, string $type): AbstractDto
+    /**
+     * @template T
+     * @psalm-param class-string<T> $type
+     *
+     * @return T
+     */
+    public function serializeDataToDto(array $data, string $type)
     {
         return $this->serializer->fromArray($data, $type);
     }
@@ -36,12 +44,24 @@ public function toArray(object $object): array
         return $this->serializer->toArray($object);
     }
 
+    /**
+     * @template T
+     * @psalm-param class-string<T> $type
+     *
+     * @return T
+     */
     public function fromArray(array $data, string $type)
     {
         return $this->serializer->fromArray($data, $type);
     }
 
-    public function deserializeXml(string $xml, string $type): SerializableInterface
+    /**
+     * @template T
+     * @psalm-param class-string<T> $type
+     *
+     * @return T
+     */
+    public function deserializeXml(string $xml, string $type)
     {
         return $this->serializer->deserialize($xml, $type, self::XML_TYPE);
     }
diff --git a/src/Serializer/SerializerInterface.php b/src/Serializer/SerializerInterface.php
index 5807679..2a8fd7f 100644
--- a/src/Serializer/SerializerInterface.php
+++ b/src/Serializer/SerializerInterface.php
@@ -4,17 +4,33 @@
 
 namespace BlueMedia\Serializer;
 
-use BlueMedia\Common\Dto\AbstractDto;
-
 interface SerializerInterface
 {
-    public function serializeDataToDto(array $data, string $type): AbstractDto;
+    /**
+     * @template T
+     * @psalm-param class-string<T> $type
+     *
+     * @return T
+     */
+    public function serializeDataToDto(array $data, string $type);
 
     public function toArray(object $object): array;
 
+    /**
+     * @template T
+     * @psalm-param class-string<T> $type
+     *
+     * @return T
+     */
     public function fromArray(array $data, string $type);
 
-    public function deserializeXml(string $xml, string $type): SerializableInterface;
+    /**
+     * @template T
+     * @psalm-param class-string<T> $type
+     *
+     * @return T
+     */
+    public function deserializeXml(string $xml, string $type);
 
     public function toXml($data): string;
 }
diff --git a/src/Transaction/Builder/TransactionDtoBuilder.php b/src/Transaction/Builder/TransactionDtoBuilder.php
index 285dc67..85dacaf 100644
--- a/src/Transaction/Builder/TransactionDtoBuilder.php
+++ b/src/Transaction/Builder/TransactionDtoBuilder.php
@@ -4,7 +4,6 @@
 
 namespace BlueMedia\Transaction\Builder;
 
-use BlueMedia\Common\Dto\AbstractDto;
 use BlueMedia\Configuration;
 use BlueMedia\Hash\HashGenerator;
 use BlueMedia\Serializer\Serializer;
@@ -12,9 +11,10 @@
 
 final class TransactionDtoBuilder
 {
-    public static function build(array $transactionData, Configuration $configuration): AbstractDto
+    public static function build(array $transactionData, Configuration $configuration): TransactionDto
     {
         $serializer = new Serializer();
+        /** @var TransactionDto $transactionDto */
         $transactionDto = $serializer->serializeDataToDto($transactionData, TransactionDto::class);
         $transactionDto->getTransaction()->setServiceId($configuration->getServiceId());
 
diff --git a/src/Transaction/Dto/TransactionDto.php b/src/Transaction/Dto/TransactionDto.php
index 7cdc88e..3b3397b 100644
--- a/src/Transaction/Dto/TransactionDto.php
+++ b/src/Transaction/Dto/TransactionDto.php
@@ -5,7 +5,7 @@
 namespace BlueMedia\Transaction\Dto;
 
 use BlueMedia\Common\Dto\AbstractDto;
-use BlueMedia\Serializer\SerializableInterface;
+use BlueMedia\Common\ValueObject\AbstractValueObject;
 use BlueMedia\Transaction\ValueObject\Transaction;
 use JMS\Serializer\Annotation\Type;
 
@@ -44,7 +44,7 @@ public function setHtmlFormLanguage(string $htmlFormLanguage): self
         return $this;
     }
 
-    public function getRequestData(): SerializableInterface
+    public function getRequestData(): AbstractValueObject
     {
         return $this->getTransaction();
     }
diff --git a/src/Transaction/ValueObject/Transaction.php b/src/Transaction/ValueObject/Transaction.php
index b8956c7..252d796 100644
--- a/src/Transaction/ValueObject/Transaction.php
+++ b/src/Transaction/ValueObject/Transaction.php
@@ -4,6 +4,7 @@
 
 namespace BlueMedia\Transaction\ValueObject;
 
+use BlueMedia\Common\ValueObject\HashableInterface;
 use BlueMedia\Serializer\SerializableInterface;
 use BlueMedia\Common\ValueObject\AbstractValueObject;
 use JMS\Serializer\Annotation\Type;
@@ -43,7 +44,7 @@
  *      "hash"
  * })
  */
-class Transaction extends AbstractValueObject implements SerializableInterface
+class Transaction extends AbstractValueObject implements SerializableInterface, HashableInterface
 {
     /**
      * Transaction service id.
diff --git a/src/Transaction/View.php b/src/Transaction/View.php
index 555cf2f..4ac5a15 100644
--- a/src/Transaction/View.php
+++ b/src/Transaction/View.php
@@ -4,13 +4,13 @@
 
 namespace BlueMedia\Transaction;
 
-use BlueMedia\Common\Dto\AbstractDto;
 use BlueMedia\Common\Enum\ClientEnum;
 use BlueMedia\Common\Util\Translations;
+use BlueMedia\Transaction\Dto\TransactionDto;
 
 final class View
 {
-    public static function createRedirectHtml(AbstractDto $transactionDto): string
+    public static function createRedirectHtml(TransactionDto $transactionDto): string
     {
         $translation = (new Translations())->getTranslation(
             $transactionDto->getHtmlFormLanguage()