Most definitions of an expert system include some reference to the ability of the system to perform at a level close to human expert performance. Yet the validation of expert systems, that is, the testing of systems so as to ascertain that they achieve an acceptable level of performance, has (with a few exceptions) been ad-hoc, informal, and in some cases of dubious value. This paper attempts to establish validation as an important concern in expert systems research and development. The problems in validating an expert system are discussed, and a number of methods for validating expert systems, both qualitative and quantitative, are presented.