The use of expert systems is a break-through in solving problems which require complex decision-making based on the past experience of an expert human being rather than relying on sequential algorithms which include simple combination of logical rules and calculations. Expert system building tools are programming systems developed to facilitate the construction of expert systems in a particular domain with less effort and in a relatively shorter time. Expert system building tools are now moving from Lisp machines to low-cost PC-based work stations, providing almost the same environment for representing, reasoning and explaining knowledge as on larger systems. This paper describes the basic characteristics and features of expert system building tools and discusses the use and the importance of these basic characteristics and features from a perspective of computer aided process planning (CAPP), requirements. Existing features are explained and evaluated to facilitate the selection and use of expert system building tools in automating the process planning task.