Understanding the role of personal motivation in addiction treatment changed with the advent of the Transtheoretical Model of intentional behavior change, a better understanding of relapse, and a shift in focus from denial to readiness. Motivation is a complex concept that covers many diverse aspects of the process of intentional behavior change. This review examines current perspectives on readiness and the stages of change, criticisms and measurement issues, and clinical applications and future research in this area. Although significant challenges remain, properly incorporating the concepts of readiness and the stages of change into addiction treatment enables providers to address the diverse needs of substance abusers and treatment seekers, supports more proactive interventions, creates a concentration on motivational enhancement, and helps researchers understand the larger process of change where addict and treatment provider meet. Better measurement, more frequent assessments, and a better understanding of the stage subtasks and how they relate to readiness and successful change are needed to deepen our understanding of motivation and its role in the treatment of addiction. (Am J Addict 2004;13:103–119)