This report describes the functions of a message switching communications system and presents an implementation in terms of the Modula programming language. In particular, the report: (1) describes a representative application of the proposed new Department of Defense high order language; (2) presents a design technique for software specification; (3) develops Modula programs for each of the message switching components; and (4) evaluates the utility of Modula as a language for the design of large parallel systems.