Acceptance and Commitment Therapy (ACT) is one of the third generation behavior therapies which has attracted most attention in recent years. ACT adopted an inductive model of treatment development guided by clearly stated philosophical assumptions. Its goal focuses on a behavioral change technology based on behavioral change processes, promoted by specific techniques (metaphors, paradox and experiential exercises), in a context of work with clients in order to clarify personal values. ACT is a contextual and functional model of clinical intervention. Up to now the strongest empirical evidence for this therapy has been obtained from correlational and experimental studies about behavioural change processes assumed by ACT. Regarding its effectiveness research, they is preliminary but very promising evidence in many clinical problems treated with ACT protocols. Some future lines of conceptual and empirical work to be solved by ACT are also revised.