AbstractThe transtheoretical model, in general, and the stages of change, in particular, have proven useful in adapting or tailoring treatment to the individual. We define the stages and processes of change and then review previous meta‐analyses on their interrelationship. We report an original meta‐analysis of 39 studies, encompassing 8,238 psychotherapy patients, to assess the ability of stages of change and related readiness measures to predict psychotherapy outcomes. Clinically significant effect sizes were found for the association between stage of change and psychotherapy outcomes (d = .46); the amount of progress clients make during treatment tends to be a function of their pretreatment stage of change. We examine potential moderators in effect size by study outcome, patient characteristics, treatment features, and diagnosis. We also review the large volume of behavioral health research, but scant psychotherapy research, that demonstrates the efficacy of matching treatment to the patient's stage of change. Limitations of the extant research are noted, and practice recommendations are advanced. © 2010 Wiley Periodicals, Inc. J Clin Psychol: In Session 67:143–154, 2011.