The very idea of mental illness is contested. Given its differences from physical illnesses, is it right to count it, and particular mental illnesses, as genuinely medical as opposed to moral matters? One debate concerns its value-ladenness, which has been used by anti-psychiatrists to argue that it does not exist. Recent attempts to define mental illness divide both on the presence of values and on their consequences. Philosophers and psychiatrists have explored the nature of the general kinds that mental illnesses might comprise, influenced by psychiatric taxonomies such as the Diagnostic and Statistical Manual and the International Classification of Diseases, and the rise of a rival biological ‘meta-taxonomy’: the Research Domain Criteria (RDoC). The assumption that the concept of mental illness has a culturally invariant core has also been questioned. This Element serves as a guide to these contested debates.