The short- and long-term health benefits of both regular physical activity and healthy eating are well established for persons of all ages. Despite publicity about these benefits and publications of activity and healthy-eating guidelines, many people struggle with implementing exercise and eating goals. With this work, we propose multiple social cognitive-motivational mechanisms by which social support can promote self-regulatory success in these domains. Specifically, we outline how interpersonal processes can improve both action planning and action control across the pre-action, action, and post-action phases of physical activity and healthy-eating goal pursuit. We suggest that close relationships offer a promising avenue for improving and promoting individual-level health behavior. We also discuss limitations, costs, and potential future interventions relating to health, social support, and interpersonal processes.