Acceptance and commitment therapy (ACT) is a third generation of cognitive-behavioral therapies. The point is to help patients to improve their psychological flexibility in order to accept unavoidable private events. Thus, they have the opportunity to invest energy in committed actions rather than struggle against their psychological events.(i) To present the ACT basic concepts and (ii) to propose a systematic review of the literature about effectiveness of this kind of psychotherapy.(i) The core concepts of ACT come from Monestès (2011), Schoendorff (2011), and Harris (2012); (ii) we conducted a systematic review of the literature using the PRISMA's criteria. The research paradigm was « acceptance and commitment therapy AND randomized controlled trial ». The bases of the MEDLINE, Cochrane and Web of science have been checked.Overall, 61 articles have been found, of which, after reading the abstracts, 40 corresponded to the subject of our study. (I) Psychological flexibility is established through six core ACT processes (cognitive defusion, acceptance, being present, values, committed action, self as context), while the therapist emphasizes on experiential approach. (II) Emerging research shows that ACT is efficacious in the psychological treatment of a wide range of psychiatric problems, including psychosis, depression, obsessive-compulsive disorder, trichotillomania, generalized anxiety disorder, post-traumatic stress disorder, borderline personality disorder, eating disorders. ACT has also shown a utility in other areas of medicine: the management chronic pain, drug-dependence, smoking cessation, the management of epilepsy, diabetic self-management, the management of work stress, the management of tinnitus, and the management of multiple sclerosis. Meta-analysis of controlled outcome studies reported an average effect size (Cohen's d) of 0.66 at post-treatment (n=704) and 0.65 (n=580) at follow-up (on average 19.2 weeks later). In studies involving comparison between ACT and active well-specified treatments, the effect size was 0.48 at post (n=456) and 0.62 at follow-up (n=404). In comparisons with waist list, treatment as usual, or placebo treatment, the effect sizes were 0.99 at post (n=248) and 0.71 at follow-up (n=176). Furthermore, ACT studies pointed out learning specific skills, such as decreasing experiential avoidance, increasing cognitive defusion, acceptance and contact with the present moment. Finally, all ACT studies showed an improved quality of life.The loss of psychological flexibility is the origin of the pain caused by psychiatric disorders and chronic diseases. This is why other studies are needed to investigate ACT's full potential.