Early in the lives of children, parental influences are strong, and interventions targeting parents are essential to behavior change. In adolescence, peers emerge as critical additions to the influence of family members; their influence can support the growth and maintenance of positive health behaviors, or it can encourage unhealthy choices. Social groups continue to feature prominently in various ways throughout adulthood. A crucial role is played by supportive social networks in the improvement and maintenance of a wide variety of health behaviors, and the availability of normative information affects health choices. Health care providers hold a good deal of power in the practitioner–patient relationship and influence their patients toward health outcomes in a variety of ways. Finally, system-level influences such as public health programs, health-related media messages, and educational interventions can help motivate individuals toward ideal health behaviors.