The formal method of interworking networks in accordance with the overall open systems interconnection (OSI) effort being pursued by both the International Organization for Standardization (ISO) and the International Telegraph and Telephone Consultative Committee (CCITT) is described. The problem is introduced, the applicable OSI concepts are defined, and the basic architectural model for interworking and the resulting requirements on protocols, relaying, addressing, and routing are discussed. The problem of interworking from the perspective of the bottom four layers of the reference model (RM)-OSI is approached in a heuristic and informal fashion. The necessary background material is presented using a rigorous set of tools is developed by the work on the RM-OSI. The concepts of interworking are further developed. Conclusions and observations concerning ongoing work are presented.>