The Transtheoretical Model (TTM) has gained widespread popularity and acceptance, yet little is known about its effectiveness as a basis for health behavior intervention. A systematic review was conducted in order to evaluate the effectiveness of TTM interventions in facilitating health-related behavior change. Thirty-five electronic databases, catalogues, and internet resources were searched for relevant studies. In addition, the bibliographies of retrieved references were scanned for further relevant publications and authors were contacted for further information where necessary. Thirty-seven randomized controlled trials, targeting seven health-related behaviors, satisfied the inclusion criteria. Overall, the methodological quality of trials was variable, and there was limited evidence for the effectiveness of stage-based interventions as a basis for behavior change or for facilitating stage progression, irrespective of whether those interventions were compared with other types of intervention or with no intervention or usual care controls. The theoretical and practical implications of these findings are discussed.