diff --git a/src/main/scala/cafebabe/ClassFile.scala b/src/main/scala/cafebabe/ClassFile.scala
index fba3e53..f97407b 100644
--- a/src/main/scala/cafebabe/ClassFile.scala
+++ b/src/main/scala/cafebabe/ClassFile.scala
@@ -28,12 +28,14 @@ class ClassFile(val className: String, parentName: Option[String] = None) extend
 
   private var fields: List[FieldInfo] = Nil
   private var methods: List[MethodInfo] = Nil
+  private var interfaces: List[InterfaceInfo] = Nil
+  private var attributes : List[AttributeInfo] = Nil
 
-  // TODO
-  private var interfacesCount: U2 = 0
-  private var interfaces: List[U1] = Nil
+  def addInterface(name: String) {
+    val nameIndex = constantPool.addClass(constantPool.addString(name))
 
-  private var attributes : List[AttributeInfo] = Nil
+    interfaces = InterfaceInfo(name, nameIndex) :: interfaces
+  }
 
   private var _srcNameWasSet = false
   /** Attaches the name of the original source file to the class file. */
@@ -140,7 +142,7 @@ class ClassFile(val className: String, parentName: Option[String] = None) extend
       accessFlags <<
       thisClass <<
       superClass <<
-      interfacesCount <<
+      interfaces.size.asInstanceOf[U2] << interfaces.reverse <<
       fields.size.asInstanceOf[U2] << fields <<
       methods.size.asInstanceOf[U2] << methods <<
       attributes.size.asInstanceOf[U2] << attributes
diff --git a/src/main/scala/cafebabe/InterfaceInfo.scala b/src/main/scala/cafebabe/InterfaceInfo.scala
new file mode 100644
index 0000000..1820a4d
--- /dev/null
+++ b/src/main/scala/cafebabe/InterfaceInfo.scala
@@ -0,0 +1,9 @@
+package cafebabe
+
+import ClassFileTypes._
+
+case class InterfaceInfo(interfaceName: String, nameIndex: U2) extends Streamable {
+  override def toStream(stream: ByteStream): ByteStream = {
+    stream << nameIndex
+  }
+}