Cryptorchidism remains a common but poorly understood anomaly affecting 2%-5% of children. Major controversies exist over the timing and type of treat ment (i. e. surgical vs hormonal therapy) and whether intervention prevents sub sequent infertility and testicular cancer. Part of the dilemma is caused by lack of understanding of the normal process of descent, despite intensive research and study since the eighteenth century, when Hunter and von Haller first described the gubernaculum (caudal genito-inguinal ligament) and processus vaginalis (Williams and Hutson 1991a). More needs to be known, not only of the normal mechanism, but also of the ways that it can be deranged to produce undescended testes. This review describes the recent results of studies on normal descent, with emphasis on our studies of the different stages of descent. As the reader will appreciate, the ever-more complex models proposed to account for normal tes ticular descent predict that undescended testes will be caused by a multitude of anomalies affecting different stages of the process. A description is given of current hypotheses of the aetiology of cryptorchidism, correlating these with studies of cryptorchid animal models. For a more detailed description of the numerous ideas about the mechanism, the reader is referred to Heyns and Hutson (1995) and other monographs (Fonkalsrud and Mengel 1981; Abney and Keel 1989)."