Acceptance and Commitment Therapy (ACT) is a relatively new form of psychotherapy, pioneered by Steven C. Hayes in the mid 1990s. It is an outgrowth of behavioral therapy and cognitive behavioral therapy (CBT), which has largely been the accepted method for treatment of conditions like depression, anxiety, and post-traumatic stress disorders. Acceptance and Commitment Therapy, like CBT, relies on the philosophy of Functional Contextualism, a school of thought suggesting that words and ideas can only be understand within context and are hence frequently misconstrued because people have individual contexts. Another influence on ACT is Relational Frame Therapy, a form of behavioral analysis examining language and learning.

CBT focuses on identifying “hot thoughts” when in the throes of an anxiety attack or deep depression, and then evaluating such thoughts to gauge how true they really are. For example, a person who is feeling unduly anxious might evaluate a thought like, “Everybody hates me,” and then list evidence as to why this is or is not true. After looking at the underlying thoughts that cause anxiety, a person evaluates whether his or her stress has been reduced. The process seems long, but after a while, people can adeptly work this process in their head, understanding that these thoughts occur but are not representative of what is really “true”. When such thoughts occur, they can be dismissed after training in CBT.

Acceptance and Commitment Therapy differs from CBT because it immediately accepts the thought, “Everybody hates me.” The thought is viewed without passion, and the statement is sometimes verbalized as, “I am having the thought that everybody hates me.” This may be repeated until the thought is defused. Hayes recognizes about 100 defusion techniques in Acceptance and Commitment Therapy.

Previous unwanted thoughts are not actively dismissed by the person undergoing Acceptance and Commitment Therapy, but are rather embraced. This is also distinctive from CBT because CBT aims to reduce unwanted and unhelpful thoughts. ACT therapists claim that the process of their therapy takes far less time than CBT, and is therefore more effective.

Mindfulness, and being present in daily living and thoughts, are particularly stressed in Acceptance and Commitment Therapy. ACT also aims to help people identify their set of inner values. Acceptance and Commitment Therapy focuses on choosing behaviors that accord with these values. ACT places emphasis on things that can be controlled, like the set of the mouth, the rapidity of breaths, or the way one’s arms and legs move.

Acceptance and Commitment Therapy prides itself on its empirical data. Since 1996, about 20 clinical studies have assessed its effectiveness in varied situations that require psychological intervention. So far, Hayes' claims have been supported by clinical trials. However, proving these claims empirically requires more study, and is sometimes a means by which other therapists reject Acceptance and Commitment Therapy. To claim a theory is empirically proven, one most rely on a far greater number of clinical trials.

Currently, Hayes and other proponents of Acceptance and Commitment Therapy teach their methods of therapies in workshops around the world. These workshops tend to be two to three days in length. Universities offering degrees in Psychology and Counseling now frequently devote a class to this therapy and other third-wave behavioral therapies.