The Concept of Diagnostic Spectra Classification has long been, and still is, a problem for psychiatry. With rare exceptions, the diagnosis of mental disorders has not been based on an understanding of etiology and pathogenesis. Instead diagnostic categories are almost entirely based on phenomenology: the signs clinicians see and the symptoms patients report. The result is that most diagnoses are little but syndromes-clusters of signs and symptoms that may or may not correspond to a disease process. This leads to “diagnostic creep,” in which diagnoses expand their boundaries into neighboring disorders and into normality. These problems also apply to the concept of a diagnostic spectrum. Similar signs and symptoms do not necessarily prove that patients have the same illness.