Outcome studies have shown that several forms of psychotherapy are effective for depression, but questions about how, why, and when these treatments work remain. The proposed study answers the call for innovative research designs that allow for more intensive, individualized data collection that can address complex questions about the dynamic nature of treatment-related change. In this R21 application, we propose a proof-of-concept study using an innovative application of experience sampling methodology (ESM) that combines the idiographic advantages of single-subject research with the statistical advantages associated with larger samples and multilevel analyses. ESM involves sampling aspects of behavior occurring in the participant's everyday environment, thereby enhancing ecological validity and reducing retrospective bias. Thirtyfour adults with primary major depressive disorder will be randomly assigned to 16 weeks of either selfsystem therapy (SST), a new short-term, empirically-supported psychotherapy for depression that targets deficits in incentive motivation and goal pursuit, or treatment-as-usual (TAU). Participants will complete a battery of individual difference and diagnostic measures at pretreatment. One week of intensive ESM will be collected at both pre- and post-treatment, during which participants will be signaled at random times during each day to answer questions about current functioning in several domains (e.g., affect, cognition, and goal pursuit) delivered via a phone-based interactive voice response (IVR) system. Throughout the 16 weeks of treatment, participants will complete similar IVR assessments on two randomly-selected days per week. The proposed study will test three main hypotheses. (1) Patients in SST will show greater increases in engagement with promotion goals and activities as well as greater increases in goal-related daily experiences of positive affect over the course of treatment compared to patients in TAU. (2) Patients in SST will show greater decreases in engagement with prevention goals and activities as well as greater decreases in goal- related daily experiences of negative affect over the course of treatment compared to patients in TAU. (3) Within the SST condition, a history of failure to pursue or achieve promotion goals and individual differences in chronic regulatory orientation (i.e., strength of orientation toward promotion and prevention goals) will moderate the effects of SST on changes in depressive symptoms and on daily experiences of promotion goal engagement. The proposed research will have a significant impact on an important public health problem and will expand our knowledge of how psychotherapy for depression works. The data to be obtained from this study will provide a sound empirical and theoretical framework for largerscale programmatic research on the nature of treatment-induced change processes in patients with depression.