diff --git a/app.hypermedia.testing.dsl/src/main/java/app/hypermedia/testing/dsl/HydraRuntimeModule.xtend b/app.hypermedia.testing.dsl/src/main/java/app/hypermedia/testing/dsl/HydraRuntimeModule.xtend
index bd7c58f..15f3c25 100644
--- a/app.hypermedia.testing.dsl/src/main/java/app/hypermedia/testing/dsl/HydraRuntimeModule.xtend
+++ b/app.hypermedia.testing.dsl/src/main/java/app/hypermedia/testing/dsl/HydraRuntimeModule.xtend
@@ -10,7 +10,7 @@ import org.eclipse.xtext.conversion.IValueConverterService
  * Use this class to register components to be used at runtime / without the Equinox extension registry.
  */
 class HydraRuntimeModule extends AbstractHydraRuntimeModule {
-        override Class<? extends IValueConverterService> bindIValueConverterService() {
+    override Class<? extends IValueConverterService> bindIValueConverterService() {
         return HydraValueConverterService
     }
 }
diff --git a/app.hypermedia.testing.dsl/src/main/java/app/hypermedia/testing/dsl/validation/HydraValidator.xtend b/app.hypermedia.testing.dsl/src/main/java/app/hypermedia/testing/dsl/validation/HydraValidator.xtend
index e875fac..cb1877c 100644
--- a/app.hypermedia.testing.dsl/src/main/java/app/hypermedia/testing/dsl/validation/HydraValidator.xtend
+++ b/app.hypermedia.testing.dsl/src/main/java/app/hypermedia/testing/dsl/validation/HydraValidator.xtend
@@ -3,6 +3,15 @@
  */
 package app.hypermedia.testing.dsl.validation
 
+import app.hypermedia.testing.dsl.hydra.HydraPackage
+import org.eclipse.xtext.validation.Check
+import java.net.URI
+import app.hypermedia.testing.dsl.hydra.NamespaceDeclaration
+import app.hypermedia.testing.dsl.core.CorePackage
+import java.net.URISyntaxException
+import app.hypermedia.testing.dsl.core.ClassBlock
+import app.hypermedia.testing.dsl.hydra.OperationBlock
+import app.hypermedia.testing.dsl.hydra.RelaxedOperationBlock
 
 /**
  * This class contains custom validation rules. 
@@ -10,16 +19,49 @@ package app.hypermedia.testing.dsl.validation
  * See https://www.eclipse.org/Xtext/documentation/303_runtime_concepts.html#validation
  */
 class HydraValidator extends AbstractHydraValidator {
-	
-//	public static val INVALID_NAME = 'invalidName'
-//
-//	@Check
-//	def checkGreetingStartsWithCapital(Greeting greeting) {
-//		if (!Character.isUpperCase(greeting.name.charAt(0))) {
-//			warning('Name should start with a capital', 
-//					HydraPackage.Literals.GREETING__NAME,
-//					INVALID_NAME)
-//		}
-//	}
-	
+    static final String INVALID_URI = 'Value is not a valid URI'
+    
+    @Check
+    def checkValidUri(NamespaceDeclaration ns) {
+        if (tryParseUri(ns.namespace) === false) {
+            error(INVALID_URI,
+                  HydraPackage.Literals.NAMESPACE_DECLARATION__NAMESPACE)
+        }
+    }
+    
+    @Check
+    def checkValidUri(ClassBlock clas) {
+        if (tryParseUri(clas.name.value) === false) {
+            error(INVALID_URI,
+                  CorePackage.Literals.CLASS_BLOCK__NAME
+            )
+        }
+    }
+    
+    @Check
+    def checkValidUri(OperationBlock clas) {
+        if (tryParseUri(clas.name.value) === false) {
+            error(INVALID_URI,
+                  HydraPackage.Literals.OPERATION_BLOCK__NAME
+            )
+        }
+    }
+    
+    @Check
+    def checkValidUri(RelaxedOperationBlock clas) {
+        if (tryParseUri(clas.name.value) === false) {
+            error(INVALID_URI,
+                  HydraPackage.Literals.RELAXED_OPERATION_BLOCK__NAME
+            )
+        }
+    }
+    
+    private def tryParseUri(String uri) {
+        try {
+            val parsed = new URI(uri)
+            return parsed.scheme !== null
+        } catch(URISyntaxException e) {
+            return false
+        }
+    }
 }
diff --git a/app.hypermedia.testing.dsl/src/test/java/app/hypermedia/testing/dsl/tests/hydra/ClassParsingTest.xtend b/app.hypermedia.testing.dsl/src/test/java/app/hypermedia/testing/dsl/tests/hydra/ClassParsingTest.xtend
new file mode 100644
index 0000000..cc90665
--- /dev/null
+++ b/app.hypermedia.testing.dsl/src/test/java/app/hypermedia/testing/dsl/tests/hydra/ClassParsingTest.xtend
@@ -0,0 +1,51 @@
+/*
+ * generated by Xtext 2.18.0
+ */
+package app.hypermedia.testing.dsl.tests.hydra
+
+import app.hypermedia.testing.dsl.core.CorePackage
+import app.hypermedia.testing.dsl.hydra.HydraScenario
+import app.hypermedia.testing.dsl.tests.HydraInjectorProvider
+import com.google.inject.Inject
+import org.eclipse.xtext.testing.InjectWith
+import org.eclipse.xtext.testing.extensions.InjectionExtension
+import org.eclipse.xtext.testing.util.ParseHelper
+import org.eclipse.xtext.testing.validation.ValidationTestHelper
+import org.junit.jupiter.api.^extension.ExtendWith
+import org.junit.jupiter.params.ParameterizedTest
+import org.junit.jupiter.params.provider.MethodSource
+
+@ExtendWith(InjectionExtension)
+@InjectWith(HydraInjectorProvider)
+class ClassParsingTest {
+    @Inject extension ParseHelper<HydraScenario>
+    @Inject extension ValidationTestHelper
+
+    @ParameterizedTest
+    @MethodSource("app.hypermedia.testing.dsl.tests.hydra.TestCases#invalidUris")
+    def void classWithInvalidUri_failsValidation(String id) {
+        // when
+        val result = '''
+            With Class <«id»> { }
+        '''.parse
+
+        // then
+        result.assertError(
+            CorePackage.Literals.CLASS_BLOCK,
+            null,
+            "Value is not a valid URI"
+        )
+    }
+
+    @ParameterizedTest
+    @MethodSource("app.hypermedia.testing.dsl.tests.hydra.TestCases#validUris")
+    def void classWithValidUri_passesValidation(String id) {
+        // when
+        val result = '''
+            With Class <«id»> { }
+        '''.parse
+
+        // then
+        result.assertNoIssues()
+    }
+}
diff --git a/app.hypermedia.testing.dsl/src/test/java/app/hypermedia/testing/dsl/tests/hydra/OperationParsingTest.xtend b/app.hypermedia.testing.dsl/src/test/java/app/hypermedia/testing/dsl/tests/hydra/OperationParsingTest.xtend
index d5b8ce0..f81b714 100644
--- a/app.hypermedia.testing.dsl/src/test/java/app/hypermedia/testing/dsl/tests/hydra/OperationParsingTest.xtend
+++ b/app.hypermedia.testing.dsl/src/test/java/app/hypermedia/testing/dsl/tests/hydra/OperationParsingTest.xtend
@@ -18,12 +18,17 @@ import app.hypermedia.testing.dsl.tests.TestHelpers
 import app.hypermedia.testing.dsl.core.ClassBlock
 import app.hypermedia.testing.dsl.Modifier
 import app.hypermedia.testing.dsl.hydra.RelaxedOperationBlock
+import org.junit.jupiter.params.ParameterizedTest
+import org.junit.jupiter.params.provider.MethodSource
+import org.eclipse.xtext.testing.validation.ValidationTestHelper
+import app.hypermedia.testing.dsl.hydra.HydraPackage
 
 @ExtendWith(InjectionExtension)
 @InjectWith(HydraInjectorProvider)
 class OperationParsingTest {
-    @Inject
+    @Inject extension
     ParseHelper<HydraScenario> parseHelper
+    @Inject extension ValidationTestHelper
 
     @Test
     def void withOperationOnTopLevel_ParsesName() {
@@ -133,4 +138,60 @@ class OperationParsingTest {
         assertThat(operationBlock).isInstanceOf(OperationBlock)
         assertThat(operationBlock.modifier).isEqualTo(Modifier.WITH)
     }
+
+    @ParameterizedTest
+    @MethodSource("app.hypermedia.testing.dsl.tests.hydra.TestCases#invalidUris")
+    def void topOperationWithInvalidUri_failsValidation(String id) {
+        // when
+        val result = '''
+            With Operation <«id»> { }
+        '''.parse
+
+        // then
+        result.assertError(
+            HydraPackage.Literals.RELAXED_OPERATION_BLOCK,
+            null,
+            "Value is not a valid URI"
+        )
+    }
+
+    @ParameterizedTest
+    @MethodSource("app.hypermedia.testing.dsl.tests.hydra.TestCases#validUris")
+    def void topOperationWithValidUri_passesValidation(String id) {
+        // when
+        val result = '''
+            With Class <«id»> { }
+        '''.parse
+
+        // then
+        result.assertNoIssues()
+    }
+
+    @ParameterizedTest
+    @MethodSource("app.hypermedia.testing.dsl.tests.hydra.TestCases#invalidUris")
+    def void operationWithInvalidUri_failsValidation(String id) {
+        // when
+        val result = '''
+            With Class some:class { With Operation <«id»> {} }
+        '''.parse
+
+        // then
+        result.assertError(
+            HydraPackage.Literals.OPERATION_BLOCK,
+            null,
+            "Value is not a valid URI"
+        )
+    }
+
+    @ParameterizedTest
+    @MethodSource("app.hypermedia.testing.dsl.tests.hydra.TestCases#validUris")
+    def void oeprationWithValidUri_passesValidation(String id) {
+        // when
+        val result = '''
+            With Class some:class { With Operation <«id»> {} }
+        '''.parse
+
+        // then
+        result.assertNoIssues()
+    }
 }
diff --git a/app.hypermedia.testing.dsl/src/test/java/app/hypermedia/testing/dsl/tests/hydra/PrefixParsingTest.xtend b/app.hypermedia.testing.dsl/src/test/java/app/hypermedia/testing/dsl/tests/hydra/PrefixParsingTest.xtend
index 550e2c8..46b94e3 100644
--- a/app.hypermedia.testing.dsl/src/test/java/app/hypermedia/testing/dsl/tests/hydra/PrefixParsingTest.xtend
+++ b/app.hypermedia.testing.dsl/src/test/java/app/hypermedia/testing/dsl/tests/hydra/PrefixParsingTest.xtend
@@ -14,11 +14,16 @@ import app.hypermedia.testing.dsl.tests.HydraInjectorProvider
 import app.hypermedia.testing.dsl.tests.TestHelpers
 import app.hypermedia.testing.dsl.hydra.NamespaceDeclaration
 import org.junit.jupiter.api.Test
+import org.eclipse.xtext.testing.validation.ValidationTestHelper
+import org.junit.jupiter.params.ParameterizedTest
+import org.junit.jupiter.params.provider.MethodSource
+import app.hypermedia.testing.dsl.hydra.HydraPackage
 
 @ExtendWith(InjectionExtension)
 @InjectWith(HydraInjectorProvider)
 class PrefixParsingTest {
     @Inject extension ParseHelper<HydraScenario>
+    @Inject extension ValidationTestHelper
 
     @Test
     def void prefix_parsesSuccesfully() {
@@ -34,4 +39,32 @@ class PrefixParsingTest {
         assertThat(namespaceDeclaration.prefix.value).isEqualTo('foaf')
         assertThat(namespaceDeclaration.namespace).isEqualTo('http://xmlns.com/foaf/0.1/')
     }
+
+    @ParameterizedTest
+    @MethodSource("app.hypermedia.testing.dsl.tests.hydra.TestCases#invalidUris")
+    def void prefixWithInvalidUri_failsValidation(String value) {
+        // when
+        val result = '''
+            PREFIX "foaf": <«value»>
+        '''.parse
+
+        // then
+        result.assertError(
+            HydraPackage.Literals.NAMESPACE_DECLARATION,
+            null,
+            "Value is not a valid URI"
+        )
+    }
+
+    @ParameterizedTest
+    @MethodSource("app.hypermedia.testing.dsl.tests.hydra.TestCases#validUris")
+    def void prefixWithValidUri_passesValidation(String value) {
+        // when
+        val result = '''
+            PREFIX "foaf": <«value»>
+        '''.parse
+
+        // then
+        result.assertNoIssues()
+    }
 }
diff --git a/app.hypermedia.testing.dsl/src/test/java/app/hypermedia/testing/dsl/tests/hydra/TestCases.xtend b/app.hypermedia.testing.dsl/src/test/java/app/hypermedia/testing/dsl/tests/hydra/TestCases.xtend
new file mode 100644
index 0000000..1cf9af6
--- /dev/null
+++ b/app.hypermedia.testing.dsl/src/test/java/app/hypermedia/testing/dsl/tests/hydra/TestCases.xtend
@@ -0,0 +1,18 @@
+package app.hypermedia.testing.dsl.tests.hydra
+
+class TestCases {
+    static def validUris() {
+        return #[
+            "http://xmlns.com/foaf/0.1/",
+            "urn:valid:urn",
+            "mailto:dsl@example.com"
+        ]
+    }
+    
+    static def invalidUris() {
+        return #[
+            "not-a-URI",
+            "12345"
+        ]
+    }
+}
\ No newline at end of file