Poor dietary and physical activity (PA) behaviors escalate risk for obesity, cancer, and other chronic diseases, and contribute to disparities. Mid-life is a vulnerable life stage when obesity rates peak and chronic diseases emerge. Neighborhood environments provide opportunities, barriers, and cues/triggers to engage in healthy or unhealthy behaviors. Overall, however, research findings on environment-behavior associations are inconsistent and effect sizes are small. This research is limited by sole focus on residential neighborhoods, failure to consider the environment?s role in within-person daily and momentary differences in behaviors, and scarce attention to identifying for whom the environment matters and under what conditions. A new approach is needed that considers the broader environment where people spend time (activity space). Our objective in the proposed study is to address misspecification of environmental exposures ubiquitous in prior research and provide a definitive test of activity-space environment explanations for between-and within-person diet and PA variations during mid-life. The central hypotheses are that activity-space environmental exposures contribute to both between- and within-person variations in dietary and PA behaviors and more strongly influence these behaviors than residential-neighborhood environments alone. Drawing on Temporal Self-Regulation Theory, we also hypothesize activity-space environmental exposures are more consequential for diet and PA when self-regulatory capacity?trait or state factors that affect a person?s ability to make efforts to regulate behavior?is diminished. An in-depth, rigorous study of 510 Latinx, African American, and White adults ages 40-64 is proposed. We will use a rich combination of cutting-edge geographically-explicit ecological momentary assessment (GEMA) methodologies: global positioning system (GPS) location tracking; smartphone-based mini-surveys of diet, PA, and state factors; and accelerometry, as well as three 24-hour dietary recalls, anthropometric measurements, and questionnaires of trait and other factors. Sophisticated routine, daily, and momentary activity-space measures will be derived based on the spatial extent of their movement, but also duration of exposure. Multiple features of the residential and activity-space environments will be measured using GIS including absolute and relative availability of healthful and unhealthful foods, walkability, recreational resource availability (e.g., parks, fitness facilities, greenness), and crime. For the first time, this innovative research will employ a dynamic environmental exposure approach using GEMA to supply evidence on the environment?s role in between- and within-person variations in diet and PA during mid-life, a pivotal time, in a racially/ethnically diverse sample. As such it will contribute to a much-needed shift in how environmental determinants of behaviors are studied, making a lasting impact on the field. Our research is significant because the results can inform new targets for lifestyle and place-based interventions to improve health during mid-life and set the stage for better later-life health.