Adolescence is a period of life that encompasses rapid physical growth and dramatic psychosocial change. Over the centuries, humans have evolved an energy balance system that is biased toward fat storage. Obesity has become the most prevalent nutritional concern in adolescents, and clearly it has a complex etiology that includes both genetic and lifestyle aspects. The greatest impact of overweight and obesity in adolescents is the appearance of related physical and psychosocial comorbidity and the tracking of these into adulthood. Thus, there is a strong imperative to treat adolescent overweight and obesity before lifestyles become entrenched and significant comorbidity intervenes. Anorexia nervosa, at the opposite end of the weight spectrum, exhibits many older evolutionary traits that attempt to protect against undernutrition. Iron and vitamin D deficiencies are common in adolescents, both of which have important nutritional aspects.