The first wave of behavior therapy countered the excesses and scientific weakness of existing nonempirical clinical traditions through empirically studied first-order change efforts linked to behavioral principles targeting directly relevant clinical targets. The second wave was characterized by similar direct change efforts guided by social learning and cognitive principles that included cognitive in addition to behavioral and emotive targets. Various factors seem to have set the stage for a third wave, including anomalies in the current literature and philosophical changes. Acceptance and Commitment Therapy (ACT) is one of a number of new interventions from both behavioral and cognitive wings that seem to be moving the field in a different direction. ACT is explicitly contextualistic and is based on a basic experimental analysis of human language and cognition, Relational Frame Theory (RFT). RFT explains why cognitive fusion and experiential avoidance are both ubiquitous and harmful. ACT targets these processes and is producing supportive data both at the process and outcome level. The third-wave treatments are characterized by openness to older clinical traditions, a focus on second order and contextual change, an emphasis of function over form, and the construction of flexible and effective repertoires, among other features. They build on the first- and second-wave treatments, but seem to be carrying the behavior therapy tradition forward into new territory.