This article describes the design and implementation of ABEST, the ADA Based Expert System Tool. ABEST is a general purpose software tool intended for use in the development of expert system applications in an ADA environment. Since it may not be clear whether it is feasible (or even desirable) to use ADA in such applications, a discussion of the advantages and disadvantages of the language in that respect is included in the Introduction. An overview of ABEST follows, illustrating its main features through a sample application in digital circuit fault diagnosis. A discussion of planned enhancements and extensions is also provided.