The purpose of this article is to review spontaneous emission from several different viewpoints, although a large part of it will be devoted to the quantum statistical theories of spontaneous emission which have been developed recently, and to discussing the interrelations among different approaches. Einstein, at the turn of the century, presented statistical arguments to determine the rate at which spontaneous emission occurs. A dynamical theory of spontaneous emission was first given by Weisskopf and Wigner [1] and very many questions were discussed concerning line shapes [1,2]. The Weisskopf-Wigner theory was later reformulated in very general terms by Heitler and Ma [3,4] and by Goldberger and Watson [5]. All these formulations have been extremely successful in atomic physics. The shifts and widths of the levels appear in a very natural way in the formulations of Heitler and Ma and Goldberger and Watson. We must add to these formulations the one by Low [6], although this is not used in atomic physics because of its complexity. Recently the Bethe-Salpeter equation has also been used to treat spontaneous emission [7]