This paper outlines a specification system for process control software, named ESPRESO, which was developed at the Nuclear Research Center, Karlsruhe, West Germany. ESPRESO is based on some new ideas, which are combined with elements taken from other systems. ESPRESO consists of a set of concepts, a specification language, a tool for the management, evaluation and validation of specifications, and the method how to use the system. Language, tool, and method are carefully adapted to the concepts. The primary aim was to demonstrate some features of a specification system which are currently not available, rather than to provide a new tool for the software market.