Objective: We conducted a meta-analysis of cognitive–behavioral treatment (CBT) studies for adult attention-deficit/hyperactivity disorder (ADHD), examining effects versus control and effects pre-to-post treatment to maximize the clinical and research utility of findings from this growing literature. Method: Eligible studies tested adults meeting criteria for Diagnostic and Statistical Manual of Mental Disorders ADHD as determined by interview or using a standardized rating scale and measured ADHD symptoms or related impairment at baseline and posttreatment. We analyzed data from 32 studies from published and unpublished sources available through December 2015. Effect size calculations included up to 896 participants. Results: Using a random effects model, we found that CBTs had medium-to-large effects from pre- to posttreatment (self-reported ADHD symptoms: g = 1.00; 95% confidence interval [CI: 0.84, 1.16]; self-reported functioning g = .73; 95% CI [0.46, 1.00]) and small-to-medium effects versus control (g = .65; 95% CI [0.44, 0.86] for symptoms, .51; 95% CI [0.23, 0.79] for functioning). Effect sizes were heterogeneous for most outcome measures. Studies with active control groups showed smaller effect sizes. Neither participant medication status nor treatment format moderated pre-to-post treatment effects, and longer treatments were not associated with better outcomes. Conclusions: Current CBTs for adult ADHD show comparable effect sizes to behavioral treatments for children with ADHD, which are considered well-established treatments. Future treatment development could focus on identifying empirically supported principles of treatment-related change for adults with ADHD. We encourage researchers to report future findings in a way that is amenable to meta-analytic review.