An ideal system of diagnostic classification implies etiology of disorders, predicts their course and prognosis, determines treatment, serves as a means of efficient communication for clinical and administrative purposes, and defines homogeneous groups of patients for treatment or research. To the extent possible, a system of classification should be based on observable and measurable characteristics and empirical research, and it should have been demonstrated to be reliable and valid in both clinical and epidemiological samples. For children and adolescents, a diagnostic system should have been tested with youth for reliability and validity and be constructed to be developmentally appropriate and allow for changes over the course of development in both the characteristics of the disorders and the symptoms shown by individual children and adolescents. The American Psychiatric Association Diagnostic and Statistical Manual of Mental Disorders (DSM) system has undergone repeated changes in diagnostic categories and criteria as the use of each version for clinical care and research led to new knowledge that was incorporated into the next version. (See First et al. 2004 for more discussion of the principles and history of the successive versions of DSM.)