In recent years, it has become increasingly recognized that many psychiatric disorders have their onset in youth. Like many experiences of childhood, these disorders can have enduring effects, and they may affect an individual's sense of satisfaction with relationships, occupation, or self and ultimately play a significant role in the development of adult psychopathology. Variations in the presentation of psychiatric diagnoses can often be attributed to an individual's developmental stage. In fact, disorders such as separation anxiety or elimination disorder represent normal behavior at an early age, although continued symptoms inappropriate to a patient's developmental level become diagnosable and thereby a focus of treatment. Despite limited research, we have effective treatments for many childhood psychiatric illnesses. Child development and child psychiatric treatment are discussed elsewhere in this volume (see Chapter 7, "Normal Child and Adolescent Development," by Gemelli, and Chapter 36, "Treatment of Children and Adolescents," by Crawford et al.).