In this paper, we discuss the concept of mental disorder from the perspective of Darwinian psychiatry. Using this perspective does not resolve all of the quandaries which philosophers of medicine face when trying to provide a general definition of disease. However, it does take an important step toward clarifying why current methods of psychiatric diagnosis are criticizable and how clinicians can improve the identification of true mental disorders. According to Darwinian psychiatry, the validity of the conventional criteria of psychiatric morbidity is dependent on their association with functional impairment. Suffering, statistical deviance, and physical lesion are frequent correlates of mental disorders but, in absence of dysfunctional consequences, none of these criteria is sufficient for considering a psychological or behavioral condition as a psychiatric disorder. The Darwinian concept of mental disorder builds from two basic ideas: (1) the capacity to achieve biological goals is the best single attribute that characterizes mental health; and (2), the assessment of functional capacities cannot be properly made without consideration of the environment in which the individual lives. These two ideas reflect a concept of mental disorder that is both functional and ecological. A correct application of evolutionary knowledge should not necessarily lead to the conclusion that therapeutic intervention should be limited to conditions that jeopardize biological adaptation. Because one of the basic aims of medicine is to alleviate human suffering, an understanding of the evolutionary foundations of the concept of mental disorder should translate into more effective ways for promoting individual and social well-being, not into the search for natural laws determining what is therapeutically right or wrong.