Relapse serves as an early warning sign of a failure to maintain desired behavioral change. Although rates of and time to relapse appear to be similar for treated adolescents and adults, this critical review of the literature on adolescent relapse begins with an overview of developmental differences between adolescent and adult substance users that need to be considered when studying relapse. A summary of rates of relapse in treated teens is followed by a review of results on the predictive validity of different relapse definitions. Next, characteristics of teens' initial relapse in terms of substances involved, situational context, and factors associated with relapse are described. Because relapse is ideally understood in the context of longer-term patterns of use, the review also discusses the extent to which early episodes of use mark clinically significant change points in post-treatment course, and how knowledge of longer-term clinical course is essential to understanding the relapse process and mechanisms underlying the maintenance of behavioral change.