A protocol mismatch occurs when heterogeneous networks try to communicate with each other. Such mismatches are inevitable due to the proliferation of a multitude of networking architectures, hardware and software on one hand, and the need for global connectivity on the other hand. Global standardization of protocols will avoid such problems, but it may take years to be agreed upon, leaving communication problems for the present. So the alternative solution of protocol conversion has been proposed. In this paper we present a systematic approach to protocol conversion using the recent theory of supervisory control of discrete event systems. We study the problem of designing a converter for a given mismatched pair of protocols, using their specifications and the specifications for the channel and the user services. We introduce the notion of converter languages, use it obtain a necessary and sufficient condition for the existence of protocol converter and present an effective algorithm for computing it whenever it exists.