As a treatise on fuzzy set theory and its applications, "Foundations of Fuzzy Systems," by R. Kruse, J. Gebhardt, and F. Klawonn, has few equals. Succinct, authoritative, and up-to-date, it covers the basic theory very thoroughly and precisely, with emphasis on those aspects of the theory that play an important role in its applications. This is especially true of the chapters dealing with the calculus of fuzzy if-then rules—a subset of fuzzy set theory that plays a central role in the applications relating to the conception and design of both control and knowledge-based systems. To view the contents of "Foundations of Fuzzy Systems" in a proper perspective, a digression is in order. First, it is important to recognize that any crisp theory can be fuzzified and hence generalized to fuzzy theory by replacing the concept of a crisp set by that of a fuzzy set.