A knowledge-based system has been constructed to act as an assistant in the design of hydraulic valves. It has been constructed on a small system (PC). This is a research installation aimed at gaining an understanding of the processes involved in developing expert design systems. It appears that an industrial version of the system developed here would be a useful addition to a standard CAD system being used for valve design. Autocad is used as the Cad system with the knowledge-based assistant developed in Autolisp which is a dialect of Lisp. A valve spool design is discussed as an example.