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growing use of the virtual machine concept has resulted in the necessity for communication between the virtual machines. The design and operation of the Virtual Machine Communication Facility is discussed as an approach to oflering such communication. The facility is an interface allowing a logical connection between two or more virtual machines. Potential applications for this facility conclude the discussion. A formal approach for communication between logically isolated virtual machines by R. M. Jensen A virtual machine (VM) is a well-defined hardware architecture that is presented to a terminal user in a somewhat illusive manner. For those users who understand the concept, it is like having an entire real computing system at their disposal. To most CMS (Conversational Monitor System) users, the system is a timesharing and program development tool, and they give little thought to the actual machine capabilities that exist. The virtual machine is an assimilation of its counterpart, the real machine, yet its users and requirements may vary widely. The requirements for communication between virtual machines may be the same as for real machines, or may be closely related to the requirements of inter-CPu task communication. This paper discusses the Virtual Machine Communication Facility (VMCF)’” as designed and implemented for Virtual Machine Facility/370 (VM/370) Release 3, Program Level Change (PLC) 8 and sub-sequent releases. VMCF is a software interface that provides a logical connection between two or more otherwise isolated vir-tual machines. The interface is not an application but is rather a facility upon which applications may be built. Presently, the im-plementation does not include a high-level language or macros; instead, the execution of functions is achieved directly through use of the VM1370 diagnose interface (conceptually similar to SVc-level programming in other operating systems). The first part of Copyright 1979 by International Business Machines Corporation. Copying is per-mitted without payment of royalty provided that (1) each reproduction is done without alteration and (2) the Journal reference and IBM copyright notice are included on the first page. The title and abstract may be used without furthe