Using a middleware when designing a new distributed application allows portability across numerous software and hardware architectures, but introduces a new layer of potential incompatibilities due to the existence of various middleware standards. POLYORB, our generic middleware, directly addresses this problem: interoperability between distribution models. It can be customized into several "personalities" (e.g. CORBA, or RMI, or Ada 95 DSA), such as Jonathan and Quarterware. It also provides simultaneous support of multiple personalities within a single middleware instance. Software components implemented using a given distribution model may thus interact with remote entities from a different distribution model. This paper identifies the functions required to implement such a middleware and maps them on an operational design.