The adoption of distributed computing has brought with it the problem of interoperability. Today, program developers need to interoperate over large and complex heterogeneous networks. Dealing with the interoperability problems that these networks can provide leads to increasing the complexity of the overall system. This complexity can be significantly reduced through the introduction of an interface definition language which adds an abstraction layer capable of handling many of the underlying problems. Two major interface definition languages being used today include CORBA's Interface Definition Language (IDL) and Microsoft's Interface Definition Language (MIDL) associated with COM/DCOM. This paper provides an overview of both interface definition languages and a comparison between them based on our experiences developing distributed systems using both. We draw some conclusions about the strengths and weaknesses of both.