diff --git a/lib/src/test/kotlin/graphql/nadel/validation/NadelUnionValidationTest.kt b/lib/src/test/kotlin/graphql/nadel/validation/NadelUnionValidationTest.kt
new file mode 100644
index 000000000..5974162df
--- /dev/null
+++ b/lib/src/test/kotlin/graphql/nadel/validation/NadelUnionValidationTest.kt
@@ -0,0 +1,253 @@
+package graphql.nadel.validation
+
+import graphql.nadel.validation.util.assertSingleOfType
+import org.junit.jupiter.api.Test
+import kotlin.test.assertTrue
+
+class NadelUnionValidationTest {
+    @Test
+    fun `no errors for valid schema`() {
+        val fixture = NadelValidationTestFixture(
+            overallSchema = mapOf(
+                "entities" to /* language=GraphQL */ """
+                    type Query {
+                        entity(id: ID!): Entity
+                    }
+                    union Entity = Human | Dog
+                    type Human {
+                        id: ID!
+                    }
+                    type Dog {
+                        name: String
+                    }
+                """.trimIndent(),
+            ),
+            underlyingSchema = mapOf(
+                "entities" to /* language=GraphQL */ """
+                    type Query {
+                        entity(id: ID!): Entity
+                    }
+                    union Entity = Human | Dog
+                    type Human {
+                        id: ID!
+                    }
+                    type Dog {
+                        name: String
+                    }
+                """.trimIndent(),
+            ),
+        )
+
+        val errors = validate(fixture)
+        assertTrue(errors.map { it.message }.isEmpty())
+    }
+
+    @Test
+    fun `union in overall schema can have fewer members than underlying schema`() {
+        val fixture = NadelValidationTestFixture(
+            overallSchema = mapOf(
+                "entities" to /* language=GraphQL */ """
+                    type Query {
+                        entity(id: ID!): Entity
+                    }
+                    union Entity = Dog
+                    type Human {
+                        id: ID!
+                    }
+                    type Dog {
+                        name: String
+                    }
+                """.trimIndent(),
+            ),
+            underlyingSchema = mapOf(
+                "entities" to /* language=GraphQL */ """
+                    type Query {
+                        entity(id: ID!): Entity
+                    }
+                    union Entity = Human | Dog
+                    type Human {
+                        id: ID!
+                    }
+                    type Dog {
+                        name: String
+                    }
+                """.trimIndent(),
+            ),
+        )
+
+        val errors = validate(fixture)
+        assertTrue(errors.map { it.message }.isEmpty())
+    }
+
+    @Test
+    fun `errors if union in overall schema declares members not in underlying schema`() {
+        val fixture = NadelValidationTestFixture(
+            overallSchema = mapOf(
+                "entities" to /* language=GraphQL */ """
+                    type Query {
+                        entity(id: ID!): Entity
+                    }
+                    union Entity = Human | Dog | Tree
+                    type Human {
+                        id: ID!
+                    }
+                    type Dog {
+                        name: String
+                    }
+                    type Tree {
+                        location: String
+                    }
+                """.trimIndent(),
+            ),
+            underlyingSchema = mapOf(
+                "entities" to /* language=GraphQL */ """
+                    type Query {
+                        entity(id: ID!): Entity
+                    }
+                    union Entity = Human | Dog
+                    type Human {
+                        id: ID!
+                    }
+                    type Dog {
+                        name: String
+                    }
+                    type Tree {
+                        location: String
+                    }
+                """.trimIndent(),
+            ),
+        )
+
+        val errors = validate(fixture)
+        assertTrue(errors.map { it.message }.isNotEmpty())
+
+        val error = errors.assertSingleOfType<NadelSchemaValidationError.UnionHasExtraType>()
+        assertTrue(error.service.name == "entities")
+        assertTrue(error.unionType.name == "Entity")
+        assertTrue(error.extraType.name == "Tree")
+    }
+
+    @Test
+    fun `cannot add types not defined in service`() {
+        val fixture = NadelValidationTestFixture(
+            overallSchema = mapOf(
+                "issues" to /* language=GraphQL */ """
+                    type Issue {
+                        id: ID!
+                    }
+                """.trimIndent(),
+                "entities" to /* language=GraphQL */ """
+                    type Query {
+                        entity(id: ID!): Entity
+                    }
+                    union Entity = Human | Dog | Issue
+                    type Human {
+                        id: ID!
+                    }
+                    type Dog {
+                        name: String
+                    }
+                    type Tree {
+                        location: String
+                    }
+                """.trimIndent(),
+            ),
+            underlyingSchema = mapOf(
+                "issues" to /* language=GraphQL */ """
+                    type Query {
+                        echo: String
+                    }
+                    type Issue {
+                        id: ID!
+                    }
+                """.trimIndent(),
+                "entities" to /* language=GraphQL */ """
+                    type Query {
+                        entity(id: ID!): Entity
+                    }
+                    union Entity = Human | Dog
+                    type Human {
+                        id: ID!
+                    }
+                    type Dog {
+                        name: String
+                    }
+                    type Tree {
+                        location: String
+                    }
+                """.trimIndent(),
+            ),
+        )
+
+        val errors = validate(fixture)
+        assertTrue(errors.map { it.message }.isNotEmpty())
+
+        val error = errors.assertSingleOfType<NadelSchemaValidationError.MissingUnderlyingType>()
+        assertTrue(error.service.name == "entities")
+        assertTrue(error.overallType.name == "Issue")
+    }
+
+    @Test
+    fun `runs type validation on unions members`() {
+        val fixture = NadelValidationTestFixture(
+            overallSchema = mapOf(
+                "issues" to /* language=GraphQL */ """
+                    type Issue {
+                        id: ID!
+                    }
+                """.trimIndent(),
+                "entities" to /* language=GraphQL */ """
+                    type Query {
+                        entity(id: ID!): Entity
+                    }
+                    union Entity = Human | Dog | Issue
+                    type Human {
+                        id: ID!
+                    }
+                    type Dog {
+                        name: String
+                    }
+                    type Tree {
+                        location: String
+                    }
+                """.trimIndent(),
+            ),
+            underlyingSchema = mapOf(
+                "issues" to /* language=GraphQL */ """
+                    type Query {
+                        echo: String
+                    }
+                    type Issue {
+                        id: ID!
+                    }
+                """.trimIndent(),
+                "entities" to /* language=GraphQL */ """
+                    type Query {
+                        entity(id: ID!): Entity
+                    }
+                    union Entity = Human | Dog
+                    type Human {
+                        id: ID!
+                    }
+                    type Dog {
+                        name: String
+                    }
+                    type Tree {
+                        location: String
+                    }
+                    type Issue {
+                        key: ID!
+                    }
+                """.trimIndent(),
+            ),
+        )
+
+        val errors = validate(fixture)
+        assertTrue(errors.map { it.message }.isNotEmpty())
+
+        val error = errors.assertSingleOfType<NadelSchemaValidationError.MissingUnderlyingField>()
+        assertTrue(error.service.name == "entities")
+        assertTrue(error.parentType.overall.name == "Issue")
+        assertTrue(error.overallField.name == "id")
+    }
+}