This paper is concerned with the development of expert systems for mechanical design and manufacturing automation. The first section provides an overview of expert systems technology and its application in the various phases of manufacturing from design to final product. The second section deals with prototype expert systems in the engineering design domain for the selection of gear drive design factors and for the design of bearing supports in power transmission systems. The next section deals with three prototype expert systems in the diagnostics and troubleshooting domain. In automobile maintenance and repair, an expert human mechanic with several man-years of training and experience has to trace diverse causes of mechanical malfunctioning of vehicles. An expert system has been developed to assist a novice in this type of mechanical diagnosis. The other diagnostic expert systems deals with the maintenance and diagnostics of process pumps used in the chemical industries. The paper concludes by discussing the impact of expert-system technology in design and manufacturing automation.