Mental disorders impose a substantial burden of illness and disability on children and adolescents. Prevalence rates of these disorders are high and the prognoses of many of the disorders are poor (Robins, 1966). There is also growing evidence of continuities between childhood and adult disorders (Ferdinand & Verhulst, 1995; Harrington, 1992; Robins, 1966). Currently, only a small proportion of the large number of children with mental disorders receive treatment, in part because of the limited number of trained clinicians available to provide help (Burns, 1996; Offord et al., 1987; Sawyer, Sarris, Baghurst, Cornish & Kalucy, 1990). As a result, in the medium term it is unlikely that treatment programs based in secondary or tertiary services will be able to provide effective help for the majority of children with mental disorders. A range of approaches is needed to identify children with unmet need and to provide effective help. This chapter focuses on three issues relevant to the unmet need for treatment of children and adolescents: (1) identification of the needs of children with mental disorders; (2) treatment of children's mental disorders; and (3) prevention of children's mental disorders. The chapter does not address a number of issues such as exploitative work practices, sexual exploitation, or the impact of wars and natural disasters on children, all of which can have a major adverse influence on children's mental health and are of increasing concern, particularly for children living in third world countries (Desjarlais, Eisenberg, Good & Kleinman, 1995).