Introduction The last decade has seen resurgence in the empirical examination of behavior therapies (Hayes, Masuda, Bissett, Luoma, & Guerrero, 2004). Treatment development and intervention research has focused on incorporating contemporary understanding of behavioral theories with the knowledge generated by seminal behavior therapy research of the 1970's. One result has been the development of and empirical support for two variants of behavioral activation: Behavioral Activation (BA; Martell, Addis, & Jacobson, 2001) and Brief Behavioral Activation for Depression (BADT; Lejuez, Hopko, & Hopko, 2001). These approaches share foundational techniques functionally defined as helping clients identify, access, and maintain contact with sources of positive reinforcement based on their goals and values. Both interventions have demonstrated effectiveness with a variety of psychiatric conditions (Cuijpers, van Straten, & Warmerdam, 2007; Ekers, Richards, & Gilbody, 2008; Hopko, Lejuez, Ruggiero, & Eifert, 2003). However, BA and BADT were developed independently and have different origins. Behavioral activation has its genesis in the contextual behavioral theory, while BADT originates from matching law theory (see Hopko et al., 2003 for detailed analysis of differences and similarities). For the purpose of exploring the mechanism of action of Cognitive Therapy for depression (CT; Beck, Rush, Shaw, & Emery, 1979), Jacobson and colleagues (1996) conducted a component analysis of the intervention. Their research provided evidence that BA, the behavioral component of CT, was as effective at reducing depressive symptomatology as the full CT intervention, and results were maintained over a two-year follow up (Gortner, Gollan, Dobson, & Jacobson, 1998). These results called into question the need for explicit cognitive interventions when treating depression and led to a number of studies that have more thoroughly examined the effectiveness of BA as a stand-alone treatment. In an effort to replicate and extend the original findings, Dimidjian and colleagues (2004) conducted a study comparing BA, CT, paroxetine (Paxil) with clinical management, and pill placebo in the treatment of depression. The results indicated that BA and paroxetine were comparable in their effectiveness and that both outperformed CT and pill placebo. As the clinical utility of BA has emerged, the authors of this early research have clarified the theoretical underpinnings of the intervention (Jacobson, Martell, & Dimidjian, 2001; Martell et al., 2001) by incorporating Ferster's (1973) theory of depression. In this theory it is assessment of the function of behavior, rather than form, which is important in facilitating clinical change. The modern theory of BA demands that clinician and client collaboratively conduct a descriptive functional analysis of the client behavior and develop a treatment plan focused on addressing client avoidance behavior in an attempt to assist him or her to engage in more active behaviors. An increase in active behaviors enables the client to come into increased contact with available reinforcers in his or her environment. As told to clients, it is not a matter of doing things when you feel like it. Rather, it is engaging in activity because the behavior will help you to accomplish goals you have set that are consistent with one's life values and elicit reinforcement (Martell et al. , 2001). As BA has become more established as a treatment for depression, questions of its effectiveness with other psychiatric and medical populations have emerged. Those suffering from Post-traumatic Stress Disorder (PTSD) and Major Depressive Disorder (MDD) are one such population. The co-morbidity of PTSD and MDD (C-P/D) has been extensively examined and research has demonstrated co-occurrence rates exceeding that which would be expected as simple coincidence (Blanchard, Buckley, Hickling, & Taylor, 1998; Bleich, Koslowsky, Dovlev, & Lerer, 1997; Dow & Kline, 1997; Kessler, Sonnega, Bromet, Hughes, & Nelson, 1995). …