Expert Systems are a rapidly growing practical application of Artificial Intelligence techniques, and opportunities for research and development in this field are likely to increase dramatically over the next few years. The purpose of this project was to gain experience in Expert System design by developing a small, but practical, diagnostic system. For the last three years I have been working at a small firm, Systems Software and Instrumentation Ltd. S.S.I. have a number of clients who use standard software packages such as 'Mailmerge'. Since there are only one or two people in the firm who know how to operate these packages, they are regularly required to spend time diagnosing clients' problems. This is usually carried out over the phone. An Expert System could enable other less experienced employees to answer these calls. Alternatively, the system could be provided with the software package itself, thus enabling clients to solve some of their problems themselves. This report describes how a prototype Expert System shell has been developed to perform diagnoses for the 'Mailmerge' program.