As a growing literature has documented applications of the Transtheoretical Model (TTM) to substance abuse, the utility or futility of such an application has been debated widely. The purpose of this paper is to critically examine the TTM, and its conceptual and empirical applications to the field of substance abuse. This review focuses not only on the stage of change dimension of the TTM, but also the processes, decisional balance, and self-efficacy dimensions, which have received less attention in earlier reviews. Particular emphasis is placed on the measurement and conceptualization of the stage of change construct. Unanswered questions and directions for future research are identified. It is concluded that, to effectively determine the TTM's applicability to substance abuse, all dimensions must be more fully developed, validated and evaluated across a range of substance abuse problems. Further, prospective studies are needed to determine the predictive utility of the TTM, and evaluation of TTM-matched interventions will help to address the model's specificity.