The use of nocturnal penile tumescence (NPT) monitoring for the differential diagnosis of erectile dysfunction has burgeoned during the last decade. This article reviews the history, development, and attempts at validation of NPT for diagnostic purposes. Problems associated with NPT are discussed, including the potential effects of unrecorded concomitant sleep parameters and variability in data scoring and diagnostic criteria. In addition, less expensive and more convenient methods of NPT monitoring are reviewed, and their strengths and weaknesses are discussed. Finally, theoretical problems related to the use of NPT for differential diagnosis of organic and psychogenic erectile dysfunctions are presented. In light of the problems associated with NPT monitoring, exclusive reliance on this technique for diagnostic purposes is discouraged. Instead, a biopsychosocial approach to the understanding and assessment of erectile dysfunction is advocated