diff --git a/core/src/main/scala/caliban/validation/Validator.scala b/core/src/main/scala/caliban/validation/Validator.scala
index 369ad7dc2..0ab5295b9 100644
--- a/core/src/main/scala/caliban/validation/Validator.scala
+++ b/core/src/main/scala/caliban/validation/Validator.scala
@@ -33,6 +33,7 @@ object Validator {
         t.kind match {
           case __TypeKind.ENUM         => validateEnum(t)
           case __TypeKind.UNION        => validateUnion(t)
+          case __TypeKind.INTERFACE    => validateInterface(t)
           case __TypeKind.INPUT_OBJECT => validateInputObject(t)
           case _                       => IO.unit
         }
@@ -545,61 +546,147 @@ object Validator {
   }
 
   private def validateInputObject(t: __Type): IO[ValidationError, Unit] = {
-    // https://spec.graphql.org/June2018/#IsInputType()
-    def isInputType(t: __Type): Either[__Type, Unit] = t.kind match {
-      case __TypeKind.LIST | __TypeKind.NON_NULL                         => t.ofType.fold[Either[__Type, Unit]](Left(t))(isInputType)
-      case __TypeKind.SCALAR | __TypeKind.ENUM | __TypeKind.INPUT_OBJECT => Right(())
-      case _                                                             => Left(t)
+    val inputObjectContext = s"""InputObject '${t.name.getOrElse("")}'"""
+
+    def noDuplicateInputValueName(inputValues: List[__InputValue], errorContext: String) = {
+      val messageBuilder = (i: __InputValue) => s"$errorContext has repeated fields: ${i.name}"
+      val explanatory =
+        "The input field must have a unique name within that Input Object type; no two input fields may share the same name"
+      noDuplicateName[__InputValue](inputValues, _.name, messageBuilder, explanatory)
     }
 
     def validateFields(fields: List[__InputValue]): IO[ValidationError, Unit] =
-      duplicateFieldName(fields) <*
-        IO.foreach(fields)(field =>
+      noDuplicateInputValueName(fields, inputObjectContext) <*
+        IO.foreach(fields)(validateInputValue(_, inputObjectContext))
+
+    t.inputFields match {
+      case None | Some(Nil) =>
+        failValidation(
+          s"$inputObjectContext does not have fields",
+          "An Input Object type must define one or more input fields"
+        )
+      case Some(fields) => validateFields(fields)
+    }
+  }
+
+  private def validateInputValue(inputValue: __InputValue, errorContext: String): IO[ValidationError, Unit] = {
+    val fieldContext = s"InputValue '${inputValue.name}' of $errorContext"
+    for {
+      _ <- doesNotStartWithUnderscore(inputValue, fieldContext)
+      _ <- onlyInputType(inputValue.`type`(), fieldContext)
+    } yield ()
+  }
+  private def validateInterface(t: __Type): IO[ValidationError, Unit] = {
+    val interfaceContext = s"Interface '${t.name.getOrElse("")}'"
+
+    def noDuplicateFieldName(fields: List[__Field], errorContext: String) = {
+      val messageBuilder = (f: __Field) => s"$errorContext has repeated fields: ${f.name}"
+      val explanatory =
+        "The field must have a unique name within that Interface type; no two fields may share the same name"
+      noDuplicateName[__Field](fields, _.name, messageBuilder, explanatory)
+    }
+
+    def validateFields(fields: List[__Field]): IO[ValidationError, Unit] =
+      noDuplicateFieldName(fields, interfaceContext) <*
+        IO.foreach(fields) { field =>
+          val fieldContext = s"Field '${field.name}' of $interfaceContext"
           for {
-            _ <- doesNotStartWithUnderscore(field)
-            _ <- onlyInputFieldType(field)
+            _ <- doesNotStartWithUnderscore(field, fieldContext)
+            _ <- onlyOutputType(field.`type`(), fieldContext)
+            _ <- IO.foreach(field.args)(validateInputValue(_, fieldContext))
           } yield ()
-        )
+        }
 
-    def duplicateFieldName(fields: List[__InputValue]): IO[ValidationError, Unit] =
-      fields
-        .groupBy(_.name)
-        .collectFirst { case (_, f :: _ :: _) => f }
-        .fold[IO[ValidationError, Unit]](IO.unit)(duplicateField =>
-          failValidation(
-            s"InputObject has repeated fields: ${duplicateField.name}",
-            "The input field must have a unique name within that Input Object type; no two input fields may share the same name"
-          )
+    t.fields(__DeprecatedArgs(Some(true))) match {
+      case None | Some(Nil) =>
+        failValidation(
+          s"$interfaceContext does not have fields",
+          "An Interface type must define one or more fields"
         )
+      case Some(fields) => validateFields(fields)
+    }
+  }
+
+  private def onlyInputType(`type`: __Type, errorContext: String): IO[ValidationError, Unit] = {
+    // https://spec.graphql.org/June2018/#IsInputType()
+    def isInputType(t: __Type): Either[__Type, Unit] = {
+      import __TypeKind._
+      t.kind match {
+        case LIST | NON_NULL              => t.ofType.fold[Either[__Type, Unit]](Left(t))(isInputType)
+        case SCALAR | ENUM | INPUT_OBJECT => Right(())
+        case _                            => Left(t)
+      }
+    }
 
-    def doesNotStartWithUnderscore(field: __InputValue): IO[ValidationError, Unit] =
-      IO.when(field.name.startsWith("__"))(
+    IO.whenCase(isInputType(`type`)) {
+      case Left(errorType) =>
         failValidation(
-          s"InputObject can't start with '__': ${field.name}",
-          """The input field must not have a name which begins with the
-characters {"__"} (two underscores)"""
+          s"${errorType.name.getOrElse("")} of $errorContext is of kind ${errorType.kind}, must be an InputType",
+          """The input field must accept a type where IsInputType(type) returns true, https://spec.graphql.org/June2018/#IsInputType()"""
         )
-      )
+    }
+  }
 
-    def onlyInputFieldType(field: __InputValue): IO[ValidationError, Unit] =
-      IO.whenCase(isInputType(field.`type`())) {
-        case Left(errorType) =>
-          failValidation(
-            s"${errorType.name.getOrElse("")} is of kind ${errorType.kind}, must be an InputType",
-            """The input field must accept a type where IsInputType(inputFieldType) returns true, https://spec.graphql.org/June2018/#IsInputType()"""
-          )
+  private def onlyOutputType(`type`: __Type, errorContext: String): IO[ValidationError, Unit] = {
+    // https://spec.graphql.org/June2018/#IsOutputType()
+    def isOutputType(t: __Type): Either[__Type, Unit] = {
+      import __TypeKind._
+      t.kind match {
+        case LIST | NON_NULL                            => t.ofType.fold[Either[__Type, Unit]](Left(t))(isOutputType)
+        case SCALAR | OBJECT | INTERFACE | UNION | ENUM => Right(())
+        case _                                          => Left(t)
       }
+    }
 
-    t.inputFields match {
-      case None | Some(Nil) =>
+    IO.whenCase(isOutputType(`type`)) {
+      case Left(errorType) =>
         failValidation(
-          s"InputObject ${t.name.getOrElse("")} does not have fields",
-          "An Input Object type must define one or more input fields"
+          s"${errorType.name.getOrElse("")} of $errorContext is of kind ${errorType.kind}, must be an OutputType",
+          """The input field must accept a type where IsOutputType(type) returns true, https://spec.graphql.org/June2018/#IsInputType()"""
         )
-      case Some(fields) => validateFields(fields)
     }
   }
 
+  private def noDuplicateName[T](
+    listOfNamed: List[T],
+    nameExtractor: T => String,
+    messageBuilder: T => String,
+    explanatoryText: String
+  ): IO[ValidationError, Unit] =
+    listOfNamed
+      .groupBy(nameExtractor(_))
+      .collectFirst { case (_, f :: _ :: _) => f }
+      .fold[IO[ValidationError, Unit]](IO.unit)(duplicate =>
+        failValidation(
+          messageBuilder(duplicate),
+          explanatoryText
+        )
+      )
+
+  private def doesNotStartWithUnderscore(field: __Field, errorContext: String) = {
+    val explanatory = s"""The field must not have a name which begins with the characters {"__"} (two underscores)"""
+    doesNotStartWithUnderscore[__Field](field, _.name, errorContext, explanatory)
+  }
+
+  private def doesNotStartWithUnderscore(inputValue: __InputValue, errorContext: String) = {
+    val explanatory =
+      s"""The input field must not have a name which begins with the characters "__" (two underscores)"""
+    doesNotStartWithUnderscore[__InputValue](inputValue, _.name, errorContext, explanatory)
+  }
+
+  private def doesNotStartWithUnderscore[T](
+    t: T,
+    nameExtractor: T => String,
+    errorContext: String,
+    explanatoryText: String
+  ): IO[ValidationError, Unit] =
+    IO.when(nameExtractor(t).startsWith("__"))(
+      failValidation(
+        s"$errorContext can't start with '__'",
+        explanatoryText
+      )
+    )
+
   case class Context(
     document: Document,
     rootType: RootType,
diff --git a/core/src/test/scala/caliban/TestUtils.scala b/core/src/test/scala/caliban/TestUtils.scala
index 81dfc425c..d76e42af9 100644
--- a/core/src/test/scala/caliban/TestUtils.scala
+++ b/core/src/test/scala/caliban/TestUtils.scala
@@ -137,5 +137,55 @@ object TestUtils {
       resolverIO.queryResolver,
       WrongMutationUnion(_ => UIO.unit)
     )
+
+    @GQLInterface
+    sealed trait InterfaceEmpty
+    object InterfaceEmpty {
+      case class A(x: String) extends InterfaceEmpty
+      case class B(y: String) extends InterfaceEmpty
+    }
+
+    case class TestEmpty(i: InterfaceEmpty)
+    val resolverEmptyInferface = RootResolver(
+      TestEmpty(InterfaceEmpty.A("a"))
+    )
+
+    @GQLInterface
+    sealed trait InterfaceWrongFieldName
+    object InterfaceWrongFieldName {
+      case class A(__name: String) extends InterfaceWrongFieldName
+      case class B(__name: String) extends InterfaceWrongFieldName
+    }
+
+    case class TestWrongFieldName(i: InterfaceWrongFieldName)
+    val resolverInferfaceWrongFieldName = RootResolver(
+      TestWrongFieldName(InterfaceWrongFieldName.A("a"))
+    )
+
+    case class WrongArgumentName(__name: String)
+
+    @GQLInterface
+    sealed trait InterfaceWrongArgumentName
+    object InterfaceWrongArgumentName {
+      case class A(x: WrongArgumentName => UIO[Unit]) extends InterfaceWrongArgumentName
+      case class B(x: WrongArgumentName => UIO[Unit]) extends InterfaceWrongArgumentName
+    }
+
+    case class TestWrongArgumentName(i: InterfaceWrongArgumentName)
+    val resolverInterfaceWrongArgumentName = RootResolver(
+      TestWrongArgumentName(InterfaceWrongArgumentName.A(_ => UIO.unit))
+    )
+
+    @GQLInterface
+    sealed trait InterfaceWrongArgumentInputType
+    object InterfaceWrongArgumentInputType {
+      case class A(x: UnionInputObjectArg => UIO[Unit]) extends InterfaceWrongArgumentInputType
+      case class B(x: UnionInputObjectArg => UIO[Unit]) extends InterfaceWrongArgumentInputType
+    }
+
+    case class TestWrongArgumentType(i: InterfaceWrongArgumentInputType)
+    val resolverInterfaceWrongArgumentInputType = RootResolver(
+      TestWrongArgumentType(InterfaceWrongArgumentInputType.A(_ => UIO.unit))
+    )
   }
 }
diff --git a/core/src/test/scala/caliban/validation/ValidationSchemaSpec.scala b/core/src/test/scala/caliban/validation/ValidationSchemaSpec.scala
index 80a5c2249..2b5b70a9e 100644
--- a/core/src/test/scala/caliban/validation/ValidationSchemaSpec.scala
+++ b/core/src/test/scala/caliban/validation/ValidationSchemaSpec.scala
@@ -13,20 +13,46 @@ object ValidationSchemaSpec
       def check(gql: GraphQL[Any], expectedMessage: String): IO[ValidationError, TestResult] =
         assertM(gql.interpreter.run, fails[ValidationError](hasField("msg", _.msg, equalTo(expectedMessage))))
 
-      suite("ValidationSchemaSpec")({
+      suite("ValidationSchemaSpec")(
         suite("InputObjects")(
           testM("name can't start with '__'") {
             check(
               graphQL(resolverWrongMutationUnderscore),
-              "InputObject can't start with '__': __name"
+              "InputValue '__name' of InputObject 'DoubleUnderscoreArgInput' can't start with '__'"
             )
           },
           testM("should only contain types for which IsInputType(type) is true") {
             check(
               graphQL(resolverWrongMutationUnion),
-              "UnionInput is of kind UNION, must be an InputType"
+              "UnionInput of InputValue 'union' of InputObject 'UnionArgInput' is of kind UNION, must be an InputType"
+            )
+          }
+        ),
+        suite("Interface")(
+          testM("must define one or more fields") {
+            check(
+              graphQL(resolverEmptyInferface),
+              "Interface 'InterfaceEmpty' does not have fields"
+            )
+          },
+          testM("field name can't start with '__'") {
+            check(
+              graphQL(resolverInferfaceWrongFieldName),
+              "Field '__name' of Interface 'InterfaceWrongFieldName' can't start with '__'"
+            )
+          },
+          testM("field argument name can't start with '__'") {
+            check(
+              graphQL(resolverInterfaceWrongArgumentName),
+              "InputValue '__name' of Field 'x' of Interface 'InterfaceWrongArgumentName' can't start with '__'"
+            )
+          },
+          testM("field argument can't be output type") {
+            check(
+              graphQL(resolverInterfaceWrongArgumentInputType),
+              "UnionInput of InputValue 'union' of InputObject 'UnionArgInput' is of kind UNION, must be an InputType"
             )
           }
         )
-      })
+      )
     })