"In the mathematics I can report no deficience, except that it be that men do not sufficiently understand the excellent use of the pure mathematics, in that they do remedy and cure many defects in the wit and faculties intellectual. For if the wit be too dull, they sharpen it; if too wandering, they fix it; if too inherent in the sense, they abstract it. " Roger Bacon (1214?-1294?) "Mathematics-the art and science of effective reasoning. " E. W. Dijkstra, 1976 "A person who had studied at a good mathematical school can do anything. " Ye. Bunimovich, 2000 This is the third book published by Kluwer based on the very successful OOPSLA workshops on behavioral semantics (the first two books were published in 1996 [KH 1996] and 1999 [KRS 1999]). These workshops fostered precise and explicit specifications of business and system semantics, independently of any (possible) realization. Some progress has been made in these areas, both in academia and in industry. At the same time, in too many cases only lip service to elegant specifications of semantics has been provided, and as a result the systems we build or buy are all too often not what they are supposed to be. We used to live with that, and quite often users relied on human intermediaries to "sort the things out. " This approach worked perfectly well for a long time.