The DSM-IV, the dominant system in use worldwide, recognizes several disorders that arise in infancy, childhood, or adolescence. These disorders are grouped into 10 categories, which are listed and briefly described in this chapter. Although there have been no large-scale national studies of adolescents that provide estimates of the prevalence of these disorders by using current diagnostic criteria and rigorous assessment methods, several studies in the United States, Puerto Rico, Canada, and New Zealand have shown consistent results. They show that between 17 and 22 percent of adolescents suffer significant developmental, emotional, or behavioral problems. Adolescence is also associated with an increase in a number of activities that are referred to as problem or at-risk behaviors. These behaviors are significant for youth development because they place the individual at risk for a variety of adverse psychological, social, and health outcomes. The prevalence rates of problem behaviors -- such as heavy cigarette smoking, heavy drinking, and marijuana and cocaine use -- are relatively high. Mental disorders play a twofold role as an influence on decision making capabilities among delinquent youth. First, core symptoms of some of the disorders can directly interfere with decision making and also lead to delinquent behavior. Second, the child, parent, family, and contextual influences with which the disorders are often associated also are relevant to adolescent decision making. Several lines of research are needed to clarify the relations of mental disorder and youths' decision making capacities as an issue in juvenile justice, including prevalence studies, longitudinal studies of the relationship of delinquency and mental illness, and empirical work that can identify the range of characteristics of adolescents that are likely to influence their decision making. 92 references