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 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 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. Key Terms: Behavioral Activation, Major Depressive Disorder, Cognitive Behavior Therapy, Efficacious Treatment ********** Major Depressive Disorder (MDD) affects approximately 121 million people worldwide (World Health Organization, 2006). In the United States, MDD has a lifetime prevalence of 10% to 25% for women and 5% to 12% for men and a point prevalence of 5% to 9% for women and 2% to 3% for men (American Psychiatric Association, 2000). It has been determined to be the leading cause of disability and the 4th leading contributor to the global burden of disease (World Health Organization, 2006). Despite both the high prevalence and serious costs of depression, up to 90% of depressed patients in primary care settings fail to receive adequate treatment, in the form of empirically supported medication regimens or in the form of empirically supported psychotherapies for depression (Center for the Advancement of Health, 2000). Of the available psychological therapies for treating MDD. Cognitive-Behavior Therapy (CBT) has emerged as the frontline treatment of choice, based on empirical findings demonstrating its efficacy and effectiveness. Dobson (1989) conducted a meta-analysis of the literature investigating Beck's Cognitive Therapy (CT) for depression (which is classified as Cognitive-Behavioral Therapy) and found that CT is more effective than general psychotherapy, Behavior Therapy, and pharmacotherapy in the treatment of MDD. Based in part on Dobson's conclusions, Chambless and colleagues (1998) labeled Cognitive Therapy (CT) for depression as a well-established treatment. Recently, a component of CBT called Behavioral Activation (BA) has received a great deal of attention. First, because as a result of a component analysis it appeared to offer a conceptually parsimonious accounting of the measured effects of CBT. Secondly it offered potential as a more efficient and thus, cost-effective treatment for MDD. Both of these observations resulted from the work of Jacobson and colleagues (1996). For this reason the National Institute of Mental Health called for greater research attention to this technique (NIMH vision statement, 2005). Although BA has a strong theoretical foundation (Lewinsohn, Sullivan, & Grosscup, 1980) and the packaged therapy from which it is derived (CBT) has strong empirical support, the studies devoted to the efficacy and effectiveness of BA as a stand-alone therapy, although encouraging, are somewhat limited. An extensive review of the PsychInfo database yielded eleven treatment relevant studies devoted to behavioral activation, two of which were case studies (Hopko, Lejuez, & Hopko, 2004; Mulick & Naugle, 2004). One of the remaining investigations constituted a follow-up examination for an earlier treatment outcome study (Gortner, Gollan, Dobson, & Jacobson, 1998). The present paper provides a critical review of the controlled studies, and provides a meta-analytic summary of their collective findings. …