This volume contains the proceedings of the XVth North American Testis Workshop, held in Louisville, Kentucky, April 7 to 10, 1999, to describe current advances in testis biology. The first two chapters provide a useful historical perspective of testis physiology and formulate compelling research questions about important aspects of sperm formation and function. This prologue sets the scene for the remainder of the volume that follows a logical progression, as the title implies, from stem cell to cell function, but that is necessarily preceded by sex determination, the quintessential requirement for there being a testis in the first place. The program for this XVth Testis Workshop evolved from recommenda tions of the scientific committee and their advice on the selection of invited speakers. The XVth Testis Workshop has a strong comparative flavor with contributions on worms (Caenorhabditis), flies (Drosophila), and chickens, which are models that permit and thereby reveal the power of genetic analy ses of the molecular mechanisms involved in spermatogenesis. A glimpse into the future is provided with information from EST, protein, and genome databases that already have an impact on progress in our understanding of the male germ cell. Although there is heavy emphasis in these chapters on cellular and molecular e, Yents, database mining and translational profiling relevant to testis function and dysfunction and assisted reproduction tech nologies are not overlooked."