This well-written paper presents many valuable insights about relationships among expert system testing methods, expert system life cycle phases, and fault types. The authors use a technique called “life-cycle mutation testing” to compare testing methods including black-box, white-box, consistency, and completeness. The example system used for evaluation is MAPS, which is a diagnostic VLSI manufacturing expert system. Thus, there are two minor limitations on the results of this paper. The first is that the discussion and testing are limited to rule-based systems. The types of errors unique to object-oriented expert systems are not discussed. The second is that the discussion about testing methods between forward chaining and backward chaining is weak. The paper is clearly written and informative, however. It uses many diagrams and tables to illustrate the test results. This paper is a must read for anyone who wants to verify and validate an expert system.