Spermatogenesis is the process by which immature male germ cells, through a complex series of events involving mitosis, meiosis, and cellular differentiation, eventually become mature spermatozoa capable of fertilizing an ovum. This process involves the developmental progression of male germ cells through a number of spermato genetic cell types, each of which is characterized by unique features of morphology, cellular associations, and specialized functions. The unique features of each germ cell type are dictated, to a large degree, by the patterns of protein expression characteristic of each cell type. This review will examine two different aspects of the regulated expression of heat shock proteins in spermatogenic cells. First, we will review studies showing that the expression of several different members of both the hsp70 as well as hsp90 families of heat shock proteins is regulated during the differentiation of these cells. Second, we will review studies which have examined the induction of hsp expression in spermatogenic cells following exposure to elevated temperatures. Next, we will review the role of the transcription factors, heat shock factor 1 (HSF1) and HSF2 in the regulation of expression of hsps in the testis. One interesting and unique function of the male reproductive system in many species is the maintenance of the testes at a temperature below that of the other tissues of the animal. The importance of precise thermoregulation of the testis is evidenced by the fact that even slight elevations of scrotal temperature are associated with infertility. The results of recent studies have suggested a potential involvement of the cellular stress response in the mechanism responsible for these inhibitory effects of elevated testis temperature on spermatogenesis. Possible mechanisms are discussed.