Psychiatry and the other mental health professions treat problematic psychological conditions that are claimed to be “mental disorders” that qualify as genuine medical disorders. Yet the question of whether or under what conditions psychological conditions such as feelings, thoughts, and actions can be genuine medical disorders remains a matter of intense controversy. To resolve whether psychological conditions can be genuine medical disorders requires an analysis of the meaning of “medical disorder.” Several standard analyses, such as that medical disorders always involve physical lesions, or medical disorders are simply undesirable bodily and mental conditions, do not explain our nuanced judgments about disorder versus nondisorder. The common argument that all mental disorders occur in the brain, therefore all mental disorders are brain diseases, is not valid and does not explain how we recognize psychological conditions as disorders in the absence of knowledge of the hypothesized brain lesions. The best analysis, and the one most cited in the mental disorder literature, appears to be the “harmful dysfunction” analysis that combines a value criterion with a scientific criterion: A medical disorder is the harmful failure of some internal mechanism to perform one of its biologically designed functions. Because psychological functions, like physical functions, have been biologically designed by natural selection, this account explains how mental disorders can be genuine medical disorders in exactly the same sense of “disorder” as other medical disorders, namely, they are both harmful failures of biologically designed functioning. This account also explains why social deviance and failure to live up to cultural ideals are generally not mental disorders, offering protection from abuse of psychiatry for social control purposes. To the degree that current psychiatric diagnostic categories are plausible attempts to identify such harmful psychological dysfunctions, they are plausibly genuine medical disorders.