In the treatment of Major Depressive Disorder (MDD), Behavioral Activation Therapy (BA) has emerged in recent years as an efficacious intervention. Derived from a component analysis of CBT, it offered at once a parsimonious explanation for the active ingredient of CBT, while demonstrating clinical efficacy as a separate treatment. Since the original investigation by Jacobson and colleagues in 1996, several well-controlled studies have been conducted, all of which converge to suggest strong support for BA as a stand-alone therapy for MDD. In this paper we review, evaluate and classify the evidence pertinent to this intervention and provide recommendations concerning its standing as a front line treatment. We conclude that the evidence is strong, the quality of research is generally very good and the effects sizes are compelling. Through both a qualitative and meta-analytic review of this evidence we also suggest the types of future studies that will establish greater confidence in BA as a front line treatment of choice therapy for Major Depressive Disorder.