diff --git a/extension.neon b/extension.neon
index 09e83bc3..8b1305cf 100644
--- a/extension.neon
+++ b/extension.neon
@@ -16,7 +16,17 @@ services:
 		show_rule_names: %phpat.show_rule_names%,
 	)
 
-    # # # # # DECLARATION RULES # # # # #
+	# # # # # DECLARATION RULES # # # # #
+
+	# ShouldBeImmutable rules
+	-
+		class: PHPat\Rule\Assertion\Declaration\ShouldBeImmutable\PropertyAssignationRule
+		tags:
+			- phpstan.rules.rule
+	-
+		class: PHPat\Rule\Assertion\Declaration\ShouldBeImmutable\MutablePropertyRule
+		tags:
+			- phpstan.rules.rule
 
 	# ShouldBeAbstract rules
 	-
@@ -42,7 +52,7 @@ services:
 		tags:
 			- phpstan.rules.rule
 
-    # # # # # RELATION RULES # # # # #
+	# # # # # RELATION RULES # # # # #
 
 	# ShouldImplement rules
 	-
diff --git a/src/Rule/Assertion/Declaration/ShouldBeImmutable/MutablePropertyRule.php b/src/Rule/Assertion/Declaration/ShouldBeImmutable/MutablePropertyRule.php
new file mode 100644
index 00000000..e134002a
--- /dev/null
+++ b/src/Rule/Assertion/Declaration/ShouldBeImmutable/MutablePropertyRule.php
@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPat\Rule\Assertion\Declaration\ShouldBeImmutable;
+
+use PHPat\Rule\Extractor\Declaration\MutablePropertyExtractor;
+use PHPStan\Node\ClassPropertyNode;
+use PHPStan\Rules\Rule;
+
+/**
+ * @implements Rule<ClassPropertyNode>
+ */
+class MutablePropertyRule extends ShouldBeImmutable implements Rule
+{
+    use MutablePropertyExtractor;
+}
diff --git a/src/Rule/Assertion/Declaration/ShouldBeImmutable/PropertyAssignationRule.php b/src/Rule/Assertion/Declaration/ShouldBeImmutable/PropertyAssignationRule.php
new file mode 100644
index 00000000..12d383e0
--- /dev/null
+++ b/src/Rule/Assertion/Declaration/ShouldBeImmutable/PropertyAssignationRule.php
@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPat\Rule\Assertion\Declaration\ShouldBeImmutable;
+
+use PHPat\Rule\Extractor\Declaration\PropertyAssignedOutOfConstructorExtractor;
+use PhpParser\Node\Expr\Assign;
+use PHPStan\Rules\Rule;
+
+/**
+ * @implements Rule<Assign>
+ */
+class PropertyAssignationRule extends ShouldBeImmutable implements Rule
+{
+    use PropertyAssignedOutOfConstructorExtractor;
+}
diff --git a/src/Rule/Assertion/Declaration/ShouldBeImmutable/ShouldBeImmutable.php b/src/Rule/Assertion/Declaration/ShouldBeImmutable/ShouldBeImmutable.php
new file mode 100644
index 00000000..bd0e0d28
--- /dev/null
+++ b/src/Rule/Assertion/Declaration/ShouldBeImmutable/ShouldBeImmutable.php
@@ -0,0 +1,46 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPat\Rule\Assertion\Declaration\ShouldBeImmutable;
+
+use PHPat\Configuration;
+use PHPat\Rule\Assertion\Declaration\DeclarationAssertion;
+use PHPat\Rule\Assertion\Declaration\ValidationTrait;
+use PHPat\Statement\Builder\StatementBuilderFactory;
+use PHPStan\Reflection\ClassReflection;
+use PHPStan\Reflection\ReflectionProvider;
+use PHPStan\Type\FileTypeMapper;
+
+abstract class ShouldBeImmutable extends DeclarationAssertion
+{
+    use ValidationTrait;
+
+    public function __construct(
+        StatementBuilderFactory $statementBuilderFactory,
+        Configuration $configuration,
+        ReflectionProvider $reflectionProvider,
+        FileTypeMapper $fileTypeMapper
+    ) {
+        parent::__construct(
+            __CLASS__,
+            $statementBuilderFactory,
+            $configuration,
+            $reflectionProvider,
+            $fileTypeMapper,
+        );
+    }
+
+    protected function applyValidation(string $ruleName, ClassReflection $subject, bool $meetsDeclaration, array $tips): array
+    {
+        return $this->applyShould($ruleName, $subject, $meetsDeclaration, $tips);
+    }
+
+    protected function getMessage(string $ruleName, string $subject): string
+    {
+        return $this->prepareMessage(
+            $ruleName,
+            sprintf('%s should be immutable', $subject)
+        );
+    }
+}
diff --git a/src/Rule/Extractor/Declaration/MutablePropertyExtractor.php b/src/Rule/Extractor/Declaration/MutablePropertyExtractor.php
new file mode 100644
index 00000000..dcce1509
--- /dev/null
+++ b/src/Rule/Extractor/Declaration/MutablePropertyExtractor.php
@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPat\Rule\Extractor\Declaration;
+
+use PhpParser\Node;
+use PHPStan\Analyser\Scope;
+use PHPStan\Node\ClassPropertyNode;
+
+trait MutablePropertyExtractor
+{
+    public function getNodeType(): string
+    {
+        return ClassPropertyNode::class;
+    }
+
+    /**
+     * @param ClassPropertyNode $node
+     */
+    protected function meetsDeclaration(Node $node, Scope $scope): bool
+    {
+        $class = $scope->getClassReflection();
+        if ($class && $class->isReadOnly()) {
+            return true;
+        }
+
+        if (!$node->isPublic() || $node->isReadonly()) {
+            return true;
+        }
+
+        return false;
+    }
+}
diff --git a/src/Rule/Extractor/Declaration/PropertyAssignedOutOfConstructorExtractor.php b/src/Rule/Extractor/Declaration/PropertyAssignedOutOfConstructorExtractor.php
new file mode 100644
index 00000000..275326d1
--- /dev/null
+++ b/src/Rule/Extractor/Declaration/PropertyAssignedOutOfConstructorExtractor.php
@@ -0,0 +1,30 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPat\Rule\Extractor\Declaration;
+
+use PhpParser\Node;
+use PHPStan\Analyser\Scope;
+use PHPStan\Node\PropertyAssignNode;
+
+trait PropertyAssignedOutOfConstructorExtractor
+{
+    public function getNodeType(): string
+    {
+        return PropertyAssignNode::class;
+    }
+
+    /**
+     * @param PropertyAssignNode $node
+     */
+    protected function meetsDeclaration(Node $node, Scope $scope): bool
+    {
+        $functionReflection = $scope->getFunction();
+        if ($functionReflection === null || $functionReflection->getName() === '__construct') {
+            return true;
+        }
+
+        return false;
+    }
+}
diff --git a/src/Test/Builder/AssertionStep.php b/src/Test/Builder/AssertionStep.php
index c2b38e83..2bd4731b 100644
--- a/src/Test/Builder/AssertionStep.php
+++ b/src/Test/Builder/AssertionStep.php
@@ -6,6 +6,7 @@
 
 use PHPat\Rule\Assertion\Declaration\ShouldBeAbstract\ShouldBeAbstract;
 use PHPat\Rule\Assertion\Declaration\ShouldBeFinal\ShouldBeFinal;
+use PHPat\Rule\Assertion\Declaration\ShouldBeImmutable\ShouldBeImmutable;
 use PHPat\Rule\Assertion\Declaration\ShouldNotBeAbstract\ShouldNotBeAbstract;
 use PHPat\Rule\Assertion\Declaration\ShouldNotBeFinal\ShouldNotBeFinal;
 use PHPat\Rule\Assertion\Relation\CanOnlyDepend\CanOnlyDepend;
@@ -18,6 +19,13 @@
 
 class AssertionStep extends AbstractStep
 {
+    public function shouldBeImmutable(): Rule
+    {
+        $this->rule->assertion = ShouldBeImmutable::class;
+
+        return new BuildStep($this->rule);
+    }
+
     public function shouldBeAbstract(): Rule
     {
         $this->rule->assertion = ShouldBeAbstract::class;
diff --git a/tests/architecture/ConfigurationTest.php b/tests/architecture/ConfigurationTest.php
index d0f2b1d8..3202bfa0 100644
--- a/tests/architecture/ConfigurationTest.php
+++ b/tests/architecture/ConfigurationTest.php
@@ -25,4 +25,11 @@ public function test_configuration_is_final(): Rule
             ->classes(Selector::classname(Configuration::class))
             ->shouldBeFinal();
     }
+
+    public function test_configuration_is_immutable(): Rule
+    {
+        return PHPat::rule()
+            ->classes(Selector::classname(Configuration::class))
+            ->shouldBeImmutable();
+    }
 }