Obesity has become so prevalent it is now considered a national and global epidemic. The limitations of pharmacologic, surgical and environmental interventions point to the critical need for behavioral programs. However, standard behavioral treatments are only minimally effective in the long term, with most participants eventually gaining back their lost weight. Weight regain can be traced to participants' difficulty following recommended diet and physical activity prescriptions, which appears tied to immutable biological drives and internal (e.g., sadness, anxiety, cravings) and external (e.g., presence of delicious foods and labor- saving devices) cues. In our lab we have found that the ability to resist these cues is bolstered by interventions that increase distress tolerance (e.g., tolerance of negative effect, food cravings), present-moment awareness of internal states and how these may be affecting behavior (i.e., metacognitive awareness), and clarity about one's personal values. In particular, we have shown that acceptance-based behavioral interventions, which incorporate these strategies, have promise for weight control. Our NIH-funded pilot study demonstrated an especially strong effect of acceptance-based strategies on weight loss for participants who reported higher levels of depression, psychological responsivity to food, and internally and externally-cued eating. The primary goal of the proposed project is to evaluate the longer-term efficacy of ABT in relation to gold standard behavioral treatment for obesity. A secondary goal is to test hypothesized mechanisms of action of the two treatments, both during active intervention and during the post-treatment weight loss maintenance phase. Building on our recent work using lab-based behavioral measures and repeatedly-administered, ecological momentary assessment (EMA) to better understand the challenges of adopting and maintaining healthful behavior choices, we will use both these measurement strategies to most accurately capture causal pathways to in-the-moment eating and physical activity lapses. EMA offers considerable advantages over retrospective self-reports which are subject to inaccuracies and biases. We also aim to evaluate moderation hypotheses stating that the superiority of ABT will be especially pronounced for those with greater mood disturbance, sensitivity to the food environment, and internal and external disinhibited eating. Our aims work towards longer-range goals of using evidence to maximize the most effective components of interventions, matching patients to treatment type, and developing real-time interventions aimed at correcting problematic eating and physical activity decisions as they occur naturalistically. Accordingly, we will randomly assign 200 overweight participants to 24 sessions of ABT or SBT, delivered over one year. All participants will be followed until 36 months post-baseline. EMA and lab-based behavioral assessments will allow more valid measurement of moderating and mediating pathways including the relationship between internal states and their impact on behavior, and how these associations are affected by treatment.