Network Implementation Language (NIL) is a high-level Programming language currently being used for the implementation of prototype communication systems. NIL is designed for writing executable architecture which can be compiled into efficient code for the different machines and run-time environments of a family of communicating products. NIL'S distinctive features include (1) high-level primitive type families supporting constructs needed for concurrent systems, (2) facilities for decomposition of a system into modules which can be dynamically installed and interconnected, (3) compile-time typestate checking--a mechanism for enhancing language security without incurring large execution-time overhead.