This paper describes some problems and opportunities associated with conceptual modeling for the kind of "open systems" we foresee must and will be increasingly recognized as a central line of computer system development. Computer applications will be based on communication between sub-systems which will have been developed separately and independently. Some of the reasons for independent development are the following: competition, different goals and responsibilities, economics, and geographical distribution. We must deal with all the problems that arise from this conceptual disparity of sub-systems which have been independently developed. Sub-systems will be open-ended and incremental ??dergoing continual evolution. There are no global objects. The only thing that all the various sub-systems hold in common is the ability to communicate with each other. In this paper we study Open Systems from the viewpoint of Message Passing Semantics, a research programme to explore issues in the semantics of communication in parallel systems such as negotiation, transaction management, problem solving, change, and self-knowledge.