Following the development of the theory of sets by Georg Cantor toward the end of the nineteenth century, interest in that theory developed rapidly until today virtually every field of mathematics has felt its impact. Notions of space and the geometry of a space, for example, have been completely revolutionized by the theory of sets. Also, the basic concepts in analysis, such as those of limit, function, continuity, derivative, and integral are now most aptly described in terms of set-theory ideas. Most important, however, has been the opportunity for new mathematical developments undreamed of fifty years ago. Thus, in companionship with the new appreciation of postulational procedures in mathematies, abstract spaces have been born, general theories of dimension and measure have been created, and the branch of mathematics called topology has undergone a spectacular growth. In short, under the influence of set theory, a considerable unification of traditional mathematics has occurred, and new mathematics has been created at an explosive rate.