diff --git a/src/Expression/ForClasses/IsA.php b/src/Expression/ForClasses/IsA.php
index bda51c46..44f41176 100644
--- a/src/Expression/ForClasses/IsA.php
+++ b/src/Expression/ForClasses/IsA.php
@@ -26,7 +26,7 @@ public function __construct(string $allowedFqcn)
 
     public function describe(ClassDescription $theClass, string $because = ''): Description
     {
-        return new Description("should inherit from: $this->allowedFqcn", $because);
+        return new Description("{$theClass->getName()} should be a $this->allowedFqcn", $because);
     }
 
     public function evaluate(ClassDescription $theClass, Violations $violations, string $because = ''): void
diff --git a/src/Expression/ForClasses/IsNotA.php b/src/Expression/ForClasses/IsNotA.php
new file mode 100644
index 00000000..04c9c31b
--- /dev/null
+++ b/src/Expression/ForClasses/IsNotA.php
@@ -0,0 +1,44 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Arkitect\Expression\ForClasses;
+
+use Arkitect\Analyzer\ClassDescription;
+use Arkitect\Expression\Description;
+use Arkitect\Expression\Expression;
+use Arkitect\Rules\Violation;
+use Arkitect\Rules\ViolationMessage;
+use Arkitect\Rules\Violations;
+
+final class IsNotA implements Expression
+{
+    /** @var class-string */
+    private string $disallowedFqcn;
+
+    /**
+     * @param class-string $disallowedFqcn
+     */
+    public function __construct(string $disallowedFqcn)
+    {
+        $this->disallowedFqcn = $disallowedFqcn;
+    }
+
+    public function describe(ClassDescription $theClass, string $because = ''): Description
+    {
+        return new Description("{$theClass->getName()} should not be a $this->disallowedFqcn", $because);
+    }
+
+    public function evaluate(ClassDescription $theClass, Violations $violations, string $because = ''): void
+    {
+        if (is_a($theClass->getFQCN(), $this->disallowedFqcn, true)) {
+            $violation = Violation::create(
+                $theClass->getFQCN(),
+                ViolationMessage::selfExplanatory($this->describe($theClass, $because)),
+                $theClass->getFilePath()
+            );
+
+            $violations->add($violation);
+        }
+    }
+}
diff --git a/tests/Unit/Expressions/ForClasses/IsATest.php b/tests/Unit/Expressions/ForClasses/IsATest.php
index cac51400..cff0bcaa 100644
--- a/tests/Unit/Expressions/ForClasses/IsATest.php
+++ b/tests/Unit/Expressions/ForClasses/IsATest.php
@@ -7,11 +7,11 @@
 use Arkitect\Analyzer\ClassDescriptionBuilder;
 use Arkitect\Expression\ForClasses\IsA;
 use Arkitect\Rules\Violations;
-use Arkitect\Tests\Unit\Expressions\ForClasses\IsOneOfTest\Animal\Dog;
-use Arkitect\Tests\Unit\Expressions\ForClasses\IsOneOfTest\Fruit\Banana;
-use Arkitect\Tests\Unit\Expressions\ForClasses\IsOneOfTest\Fruit\CavendishBanana;
-use Arkitect\Tests\Unit\Expressions\ForClasses\IsOneOfTest\Fruit\DwarfCavendishBanana;
-use Arkitect\Tests\Unit\Expressions\ForClasses\IsOneOfTest\Fruit\FruitInterface;
+use Arkitect\Tests\Unit\Expressions\ForClasses\IsATest\Animal\Dog;
+use Arkitect\Tests\Unit\Expressions\ForClasses\IsATest\Fruit\Banana;
+use Arkitect\Tests\Unit\Expressions\ForClasses\IsATest\Fruit\CavendishBanana;
+use Arkitect\Tests\Unit\Expressions\ForClasses\IsATest\Fruit\DwarfCavendishBanana;
+use Arkitect\Tests\Unit\Expressions\ForClasses\IsATest\Fruit\FruitInterface;
 use PHPUnit\Framework\TestCase;
 
 final class IsATest extends TestCase
@@ -48,7 +48,7 @@ public function test_it_should_have_no_violation_when_it_extends(): void
         self::assertEquals(0, $violations->count());
     }
 
-    public function test_it_should_have_violation_if_it_doesnt_extend(): void
+    public function test_it_should_have_violation_when_it_doesnt_extend(): void
     {
         $interface = FruitInterface::class;
         $isA = new IsA($interface);
@@ -62,12 +62,12 @@ public function test_it_should_have_violation_if_it_doesnt_extend(): void
 
         self::assertEquals(1, $violations->count());
         self::assertEquals(
-            "should inherit from: $interface",
+            "Dog should be a $interface",
             $isA->describe($classDescription, '')->toString()
         );
     }
 
-    public function test_it_should_have_violation_if_it_doesnt_implement(): void
+    public function test_it_should_have_violation_when_it_doesnt_implement(): void
     {
         $class = Banana::class;
         $isA = new IsA($class);
@@ -81,19 +81,19 @@ public function test_it_should_have_violation_if_it_doesnt_implement(): void
 
         self::assertEquals(1, $violations->count());
         self::assertEquals(
-            "should inherit from: $class",
+            "Dog should be a $class",
             $isA->describe($classDescription, '')->toString()
         );
     }
 }
 
-namespace Arkitect\Tests\Unit\Expressions\ForClasses\IsOneOfTest\Animal;
+namespace Arkitect\Tests\Unit\Expressions\ForClasses\IsATest\Animal;
 
 final class Dog
 {
 }
 
-namespace Arkitect\Tests\Unit\Expressions\ForClasses\IsOneOfTest\Fruit;
+namespace Arkitect\Tests\Unit\Expressions\ForClasses\IsATest\Fruit;
 
 interface FruitInterface
 {
diff --git a/tests/Unit/Expressions/ForClasses/IsNotATest.php b/tests/Unit/Expressions/ForClasses/IsNotATest.php
new file mode 100644
index 00000000..af7d470d
--- /dev/null
+++ b/tests/Unit/Expressions/ForClasses/IsNotATest.php
@@ -0,0 +1,112 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Arkitect\Tests\Unit\Expressions\ForClasses;
+
+use Arkitect\Analyzer\ClassDescriptionBuilder;
+use Arkitect\Expression\ForClasses\IsNotA;
+use Arkitect\Rules\Violations;
+use Arkitect\Tests\Unit\Expressions\ForClasses\IsNotA\Animal\Dog;
+use Arkitect\Tests\Unit\Expressions\ForClasses\IsNotA\Fruit\Banana;
+use Arkitect\Tests\Unit\Expressions\ForClasses\IsNotA\Fruit\CavendishBanana;
+use Arkitect\Tests\Unit\Expressions\ForClasses\IsNotA\Fruit\DwarfCavendishBanana;
+use Arkitect\Tests\Unit\Expressions\ForClasses\IsNotA\Fruit\FruitInterface;
+use PHPUnit\Framework\TestCase;
+
+final class IsNotATest extends TestCase
+{
+    public function test_it_should_have_no_violation_when_it_doesnt_extend(): void
+    {
+        $interface = FruitInterface::class;
+        $isNotA = new IsNotA($interface);
+        $classDescription = (new ClassDescriptionBuilder())
+            ->setFilePath('src/Foo.php')
+            ->setClassName(Dog::class)
+            ->build();
+
+        $violations = new Violations();
+        $isNotA->evaluate($classDescription, $violations, '');
+
+        self::assertEquals(0, $violations->count());
+    }
+
+    public function test_it_should_have_no_violation_when_it_doesnt_implement(): void
+    {
+        $class = Banana::class;
+        $isNotA = new IsNotA($class);
+        $classDescription = (new ClassDescriptionBuilder())
+            ->setFilePath('src/Foo.php')
+            ->setClassName(Dog::class)
+            ->build();
+
+        $violations = new Violations();
+        $isNotA->evaluate($classDescription, $violations, '');
+
+        self::assertEquals(0, $violations->count());
+    }
+
+    public function test_it_should_have_violation_when_it_implements(): void
+    {
+        $interface = FruitInterface::class;
+        $isNotA = new IsNotA($interface);
+        $classDescription = (new ClassDescriptionBuilder())
+            ->setFilePath('src/Foo.php')
+            ->setClassName(CavendishBanana::class)
+            ->addInterface($interface, 10)
+            ->build();
+
+        $violations = new Violations();
+        $isNotA->evaluate($classDescription, $violations, '');
+
+        self::assertEquals(1, $violations->count());
+        self::assertEquals(
+            "CavendishBanana should not be a $interface",
+            $isNotA->describe($classDescription, '')->toString()
+        );
+    }
+
+    public function test_it_should_have_violation_when_it_extends(): void
+    {
+        $class = Banana::class;
+        $isNotA = new IsNotA($class);
+        $classDescription = (new ClassDescriptionBuilder())
+            ->setFilePath('src/Foo.php')
+            ->setClassName(DwarfCavendishBanana::class)
+            ->addExtends($class, 10)
+            ->build();
+
+        $violations = new Violations();
+        $isNotA->evaluate($classDescription, $violations, '');
+
+        self::assertEquals(1, $violations->count());
+        self::assertEquals(
+            "DwarfCavendishBanana should not be a $class",
+            $isNotA->describe($classDescription, '')->toString()
+        );
+    }
+}
+
+namespace Arkitect\Tests\Unit\Expressions\ForClasses\IsNotA\Animal;
+
+final class Dog
+{
+}
+
+namespace Arkitect\Tests\Unit\Expressions\ForClasses\IsNotA\Fruit;
+
+interface FruitInterface
+{
+}
+
+class Banana implements FruitInterface
+{
+}
+
+class CavendishBanana extends Banana
+{
+}
+
+final class DwarfCavendishBanana extends CavendishBanana
+{
+}