Medical conditions and disorders must be carefully defined both for research and for clinical practice. The most widely used definitions for alcohol use disorders are those determined by editions of the Diagnostic and Statistical Manual of Mental Disorders (DSM) of the American Psychiatric Association and the International Classification of Diseases (ICD) of the World Health Organization. Alcoholism treatment studies, human genetics studies, and epidemiology all rely on these definitions, which constitute a near-universal feature of research on alcoholism. Studies consistently show high reliability for DSM-IV and ICD-10 alcohol dependence but lower reliability for alcohol abuse/harmful use. Validity studies indicate that DSM-IV and ICD-10 alcohol dependence diagnoses have good validity, but the validity for alcohol abuse/harmful use is much lower. The hierarchical relationship of alcohol abuse to dependence may contribute to the reliability and validity problems of abuse, an issue likely to be addressed when work begins on DSM-V.