Unlike most domestic livestock species, sheep are widely known as animals with marked seasonality of breeding activity. The annual cycle of the daily photoperiod has been identified as the determinant factor of this phenomenon, while environmental temperature, nutritional status and social interactions may modulate it. The aim of this paper is to review the current state of knowledge of reproductive seasonality in sheep. Following general consideration of the importance of seasonal breeding as a reproductive strategy for the survival of the species, the paper describes manifestations of seasonality in both the ram and the ewe. Both determinant and modulating factors are discussed, and special emphasis is given to the neuroendocrine base of photoperiodic regulation of seasonal breeding. Other aspects such as the role of melatonin, the progestogens and ram effects with or without anestrus period are also reviewed.