Objective: The primary aim of this study was to assess the overall effectiveness of and dropout from individual and group outpatient cognitive behavioral therapy (CBT) for adults with a primary diagnosis of unipolar depressive disorder in routine clinical practice. Method: We conducted a random effects meta-analysis of 34 nonrandomized effectiveness studies on outpatient individual and group CBT for adult unipolar depressive disorder. Standardized mean gain effect sizes are reported for end-of-treatment and 6-month follow-up effects for depression severity, dysfunctional cognitions, general anxiety, psychological distress, and functional impairment. The mean dropout rate from CBT is reported. We benchmarked our results against high-quality randomized controlled trials (RCTs). Results: Outpatient CBT was effective in reducing depressive severity in completer (d = 1.13) and intention-to-treat (ITT) samples (d = 1.06). Moderate to large posttreatment effect sizes (d = 0.67-0.88) were found for secondary outcomes. The weighted mean dropout rate was 24.63%. Posttreatment gains for depression were maintained at 6 months after completion of therapy. Effect sizes for depression were inferior to those of benchmark RCTs. Conclusions: Although clinical practice patients show lesser improvements in depressive symptoms than RCT patients, individual and group outpatient CBT can be effectively transported to routine clinical practice. The considerable treatment dropout rate, especially in individual CBT, must be improved. The small number of available studies and low quality of some reports stress the need for high-quality effectiveness studies. (Contains 2 tables and 2 figures.)