Health psychology is the branch of psychology that studies how biological, psychological, and social factors influence health and illness. It applies psychological knowledge to the prevention and treatment of physical disease and to the promotion of healthy behavior.

Topics include the effects of stress on the immune system, the role of personality and coping in recovery from illness, adherence to medical regimens, pain management, and the design of interventions that reduce smoking, improve diet, and increase physical activity. Health psychologists work in hospitals, clinics, public-health agencies, and research settings. They collaborate with physicians, nurses, and other professionals to address the behavioral side of medical problems.

The field rests on the biopsychosocial model, which treats health as the product of interacting biological processes, individual behavior and cognition, and social context. Chronic diseases that are heavily influenced by lifestyle—heart disease, diabetes, many cancers—are central concerns. Interventions range from individual counseling to large-scale public campaigns.

Medicine treats the body. Health psychology addresses the behaviors and mental processes that help determine whether the body stays well or becomes ill.