Annual cycles in many functions have been demonstrated in practically all groups of vertebrates. Starting with the work of Rowan (1925) in birds and of Baker and Ranson (1932) and of Bissonette (1932) in mammals, it has been shown that the photoperiod (i.e., the light fraction of the 24-hr day and its seasonal changes) influences the annual cycle, especially in those species living in medium or higher latitudes. In recent years, much progress has been made in analyzing the time-measuring mechanisms involved, as well as in unveiling the physiological mechanisms that participate in the seasonal changes and their regulation by photoperiod. Most of this work has been concerned with the cycle in reproduction, and thus, this function is discussed here in greatest detail. Since there seem to be definite differences in the physiological mechanisms in different vertebrate groups, the discussion deals with these groups separately. Because of the author’s bias, emphasis is placed on the situation in mammals.