A classification system can benefit disturbed children enormously by bringing to bear a wealth of knowledge and experience. This can make all the difference between an inadequate consultation and a precise formulation of the nature and extent of a child's difficulties, their cause, the likely outcome and a realistic treatment plan. However, inappropriate application of a diagnostic label that has little validity could do more harm than good, and classification systems can be misused. This paper discusses, with examples, issues particular to childhood and adolescence that diagnostic systems need to address if they are to be useful. It considers different solutions applied by the two most widely used schemes, the International Classification of Diseases (ICD–10; World Heath Organization, 1992) and the Diagnostic and Statistical Manual of Mental Disorders (DSM–IV; American Psychiatric Association, 1994). Finally, the types of criteria used to validate categories are discussed.