Obesity prevalence can be reduced by individual-focused lifestyle interventions as these interventions are generally effective at helping people lose weight. However, some people who engage in these interventions lose less weight than others. This variability in weight loss may be influenced by psychological factors, such as perceived stress, that play a key role in behavior change. Fluctuations in daily stress and ability to cope with this stress (i.e., coping) can negatively impact behavior, and potentially initial weight loss. Nevertheless, successfully assessing this relationship on the day-to-day level has been difficult and with limited assessment especially among participants who are trying to change behavior (e.g., behavioral weight loss program). These limitations can be addressed by ecological momentary assessment (EMA) methodology which assesses these daily psychological constructs and behaviors in real-time and across multiple days. Intensive longitudinal assessment with EMA of these psychological factors and daily behavior can inform the design of adaptive interventions that can intervene in real-time to change behavior and improve weight loss success. To this end, the overarching aim of this study is to elucidate mechanisms of stress and related concepts with subsequent health behaviors during a behavioral weight loss intervention. To thoroughly assess the mechanism, this study will conduct EMA measures at multiple timepoints within diverse participants who have engaged in a community-based weight management program. Specifically, we will examine the association between variability in individual stress and weight loss across a behavioral weight loss program (Specific Aim 1). We will test the hypothesis that high variability in daily stress will be related to less weight loss (Hypothesis 1a), and less physical activity and sleep across the initial (3-months) and full program (6-months, Hypothesis 1b). We will also examine the association between variability in daily coping and positive affect, and weight loss across the behavioral weight loss program (Specific Aim 2). We will test the hypothesis that individuals with high variability in coping and positive affect will be related to less weight lost across the initial and full program (Hypothesis 2). The results from our study will advance our understanding of stress, coping, and affect during a weight loss program, and lead to the development of an ecological momentary intervention to improve stress and mental health while engaging in a behavioral weight loss program supporting long-term behavior change and weight loss success.