Acceptance and commitment therapy (ACT, which is said as one word) is a practice that guides individuals who experience negative mental and behavioral patterns (e.g., anxiety) to accept and process those patterns rather than avoid experiencing them. ACT is grounded in relational frame theory, which is a modern behavioral analytic approach that focuses on language and cognition. The linguistic process thorough which individuals interact with their environments can affect psychological functioning, causing them to frame a situation as good or bad. ACT has been described as one of the third-wave behavioral therapies. This entry explores the constituents, practice, and effectiveness of ACT.