The notion that fundamental equations governing the motions of physical systems are invariant under the time reversal transformation ("T") has been an important, but often subliminal, element in the development of theoretical physics. It serves as a powerful and useful tool in analyzing the structure of matter at all scales, from gases and condensed matter to subnuclear physics and the quantum theory of fields. The assumption of invariance under "T" was called into question, however, by the 1964 discovery that a closely related assumption, that of "CP" invariance (where "C" is charge...