A revolution occurred within the psychiatric profession in the early 1980s that rapidly transformed the theory and practice of mental health in the United States. In a very short period of time, mental illnesses were transformed from broad, etiologically defined entities that were continuous with normality to symptom-based, categorical diseases. The third edition of the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders (DSM-III) was responsible for this change. The paradigm shift in mental health diagnosis in the DSM-III was neither a product of growing scientific knowledge nor of increasing medicalization. Instead, its symptom-based diagnoses reflect a growing standardization of psychiatric diagnoses. This standardization was the product of many factors, including: (1) professional politics within the mental health community, (2) increased government involvement in mental health research and policymaking, (3) mounting pressure on psychiatrists from health insurers to demonstrate the effectiveness of their practices, and (4) the necessity of pharmaceutical companies to market their products to treat specific diseases. This article endeavors to explain the origins of DSM-III, the political struggles that generated it, and its long-term consequences for clinical diagnosis and treatment of mental disorders in the United States.