Psychiatric diagnosis has proven far more complicated than anyone believed 50 years ago when the first edition of the Diagnostic and Statistical Manual saw the light of day. Psychiatry has always had a nosology, or classification of diseases, beginning with the rough categories established in the early nineteenth century. Between the late nineteenth and mid-twentieth centuries, psychiatric classification was shaped by individual authorities, in particular by the great German nosologist Emil Kraepelin.The DSM series of the American Psychiatric Association grew out of a desire to create an American system in line with the International Classification of Diseases series issued by the World Health Organization. In some ways its evolution represents progress towards a genuine medical model of classification. Yet in other respects the series has been hijacked by consensus and horse-trading, becoming a political rather than scientific document. Despite enormous investments of time, thought and academic firepower, the process of establishing a reliable nosology of psychiatric illness continues to elude our grasp.