Functional Analytic Psychotherapy is based on the principles of radical behaviorism and is part of the so-called Clinical Behavior Analysis movement (Dougher & Hayes, 2000). It emphasizes the functional analysis of client behavior during sessions and proposes the use of natural reinforcement and the shaping of therapeutic relationships, starting from the assumption that a functional equivalence exists between clinic environment and the daily life of client. This article describes its characteristics and the method of intervention, showing clinical examples of client and therapist behaviors. Recent developments in application areas, ongoing studies about effectiveness and future evolution are also described.