This is the introductory article to a special series in Cognitive and Behavioral Practice on Acceptance and Commitment Therapy (ACT). Instead of each article herein reviewing the basics of ACT, this article contains that review. This article provides a description of where ACT fits within the larger category of cognitive behavior therapy (CBT): CBT is an overarching term for a whole cluster of therapies, and ACT is one of many forms of CBT. Functional contextualism and how it informs ACT is briefly reviewed. The behavior analytic account of cognition that informs ACT, relational frame theory (RFT), and rule-governed behavior are covered. Psychological flexibility and the 6 resulting psychological processes of change (acceptance, defusion, being present, self as context, values, and committed action) are described. The empirical support for ACT and its related model are presented. Finally, characteristics of the ACT model, including its therapeutic approach, desired outcomes, and processes of change, are reviewed.