ABSTRACT Most adults in the U.S. fail to meet national physical activity (PA) recommendations, and minorities are less likely to meet these recommendations than non-Hispanic Whites. Physical inactivity and obesity are major risk factors for cardiovascular disease, certain cancers, diabetes, and stroke and are important contributors to preventable morbidity and mortality in the U.S. Social environments are widely recognized to have an important impact on PA, yet social contexts remain understudied in intervention research. The goal of the proposed research is to evaluate the effectiveness of a 6-month behavioral dyadic intervention to promote positive and sustained change in PA among inactive predominantly Latina and African American women in Houston, TX. Dyads will first be randomly assigned to the dyadic intervention or to an individual condition. Within the individual condition, one woman from each dyad will subsequently be randomized to the individual intervention and the other woman to a health education control. The dyadic and individual interventions will consist of telephone-based health coaching, a Fitbit, and health education newsletters to enhance motivation and behavioral skills for increasing PA. The health coaching for the dyadic intervention additionally focuses on building participants' capacity to be a supportive partner by training dyads in positive communication strategies and offering support in a non-judgmental and empathetic way. The health education control will consist of a Fitbit and health education newsletters. Study participants will include 500 predominantly Latina and African American inactive women recruited and enrolled as dyads (e.g., family or friends; n=250 dyads). The intervention expands upon a pilot randomized trial conducted by the investigative team that showed preliminary evidence of support and also identified areas for improvement. Participants will be assessed at baseline, 6 months, and 12 months after baseline to evaluate both intermediate and long-term effects. The primary outcome is change in minutes per week of moderate-intensity PA. Lower body strength, anthropometric measures (i.e., BMI, waist circumference), sedentary behavior, mean daily steps, and blood pressure are secondary outcomes. Autonomous motivation, social support, autonomy support, self-efficacy, and outcome expectancies will be examined as potential mediators of changes in PA. The proposed research is innovative in its comparison of a dyadic intervention against an individual intervention and in its emphasis on dyadic social processes in addition to standard behavior change strategies. The intervention explicitly targets existing social networks to foster social environments supportive of healthy behavior change that will persist beyond the intervention period. This research is expected to yield critical insight regarding whether and how features of social contexts can be modified to support healthy lifestyle change as a means of addr...