The global epidemic of overweight and obesity results from individual factors and from meso- and macro environmental factors. Adolescents are especially vulnerable to unhealthy changes in life style. One major influence on health behavior in this age group is represented by friends in peer groups. Young people tend to select and mimic each other with respect to both eating behavior and physical exercise. Peer groups are a close and important source of influence on health behavior and social media strengthens this influence. New research and interventions should take account of the influence of the peer group and social media.