This digest describes and defines the various components of an expert system, e.g., a computerized tool designed to enhance the quality and availability of knowledge required by decision makers. It is noted that expert systems differ from conventional applications software in the following areas: (1) the existence of the expert systems shell, or interpreter; (2) the presence of a "knowledge base," or system of related concepts that enables the computer to approximate human judgment; and (3) the sophistication of the user interface. The purpose and functions of these three concepts are discussed in detail, and the computer's operating system is defined in terms of its important role in the implementation of the expert system. Eight additional readings are suggested. (MAB)