The Adolescent Treatment Models initiative, a 10-site, multimodality, prospective study, was designed to evaluate adolescent substance abuse treatment outcomes and to assess the relative efficacy of different treatment models. Based upon longitudinal data gathered at multiple assessment points using a standardized instrument, treatment outcome trajectories were determined for a cohort of 1,057 adolescents from entry into substance abuse treatment until 12 months post-intake. Client outcomes on substance use and program effectiveness were explored across individual treatment programs and levels of care. Strong treatment effects, defined as a significant reduction in alcohol and other drug use at three months post-intake, were found. The reductions of greatest magnitude in relation to pretreatment use occurred among adolescents in residential treatment. Within level of care, few significant differences in treatment effects were found between programs. Relapse effects, defined as an increase in substance use at 12 months relative to three months, were observed across nearly all programs, but varied in relation to treatment modality. This is most evident among those entering residential treatment, with the highest rate of relapse occurring among adolescents in long-term residential treatment care. Despite strong evidence of treatment effectiveness, continuing care is vital to maintenance of treatment benefit.