Acceptance and commitment therapy (ACT) is a modern form of cognitive behavioral therapy based on a distinct philosophy and basic science of cognition. This article reviews the core features of ACT's theoretic model of psychopathology and treatment and its therapeutic approach. It provides a systematic review of randomized controlled trials (RCTs) evaluating ACT for depression and anxiety disorders. Summarizing 36 RCTs, ACT appears to be more efficacious than waitlist conditions and treatment-as-usual, with largely equivalent effects relative to traditional cognitive behavioral therapy. Evidence indicates that ACT treatment outcomes are mediated through increases in psychological flexibility, its theorized process of change.