It is now well known that the failure of the processus vaginalis (PV) to close off following descent of the testis not only accounts for nearly all inguinal hernias of childhood [Rothenberg et al, 1953], but also for other common congenital conditions of childhood such as scrotal hydroceles and encysted hydroceles of the cord. Despite the direct relationship of the PV to these conditions and its intimate association with the mechanism of testicular descent, surprisingly little is known about this important structure. It is not known, for instance, how the PV develops, what precise role it plays in testicular descent, what makes it close, or exactly when it closes. The purpose of this paper is to review the past and present literature on the PV, examining specifically its role in testicular descent, its postulated modes of development, and the mechanisms of its closure and disappearence following descent of the testis.