The role of nocturnal penile tumescence monitoring in helping to distinguish psychogenic from organic impotence has been the subject of research for several decades. Tumescence monitoring alone, while it does provide useful information, imposes limitations on the diagnostic inferences that can be drawn concerning the adequacy of erectile function. This led to the development of nocturnal penile tumescence and rigidity monitoring (NPTR). Although there is no single test that enables physicians to diagnose the precise etiology and degree of impotence, NPTR provides useful information in a rather noninvasive and inexpensive manner allowing one to direct patients to the appropriate treatment.