This book presents the architecture model for an effective demand migration framework (DMF) by applying distributed middleware technologies. DMF establishes a context for design and implementation of communication systems providing persistent synchronous message-passing communication model, where messages are delivered on demand. We apply DMF to derive a demand migration system (DMS) connecting the execution nodes of a heterogeneous distributed system called GIPSY (general intensional programming system). GIPSY exposes a demand-driven execution model with essence of distributiveness. Here, we describe both design and implementation of that DMS and survey test results. We also explore the distributed middleware technologies JINI, CORBA, DCOM and .Net Remoting, those being the implementation backbone for our DMS. Note that the book covers only the demand-migration aspects and does not deal with load balancing and efficiency aspects. This book represents the blending of two viewpoints: that of the practitioner, whose main goal is to build high-quality software, and that of the researcher, who strives to make a discovery or invent new aspects of computing.