The failure of the adolescent medical patient to adhere to prescribed medical treatment is one of the major reasons for psychiatric consultation in pediatric medical settings. This article reviews the developmental issues that interfere with treatment adherence in adolescents with chronic physical illness as well as the importance of co-morbid psychiatric disorders. Incomplete adherence is conceptualized as a clinical phenomenon that can be understood by considering both developmental and psychopathological factors that influence health care behavior. Interventions designed to enhance treatment adherence need to consider both developmental issues and psychiatric co-morbidity in order to improve outcome.