The widely held belief that the diagnosis of mental disorder is a matter exclusively for value-free science has been much reinforced by recent dramatic advances in the neurosciences. In this lecture I will use a detailed case study of delusion and spiritual experience to indicate to the contrary that values come into the diagnosis of mental disorders directly through the language of the diagnostic criteria adopted in such scientifically–grounded classifications as the American Psychiatric Association’s DSM (Diagnostic and Statistical Manual). Various competing interpretations of the importance of values in psychiatric diagnosis will be considered. Interpreted through the lens of the Oxford tradition of linguistic-analytic philosophy, however, diagnostic values in psychiatry are seen to reflect the complex and often conflicting values of real people. This latter interpretation has the direct consequence that there is a need for processes of assessment in psychiatry that are equally values-based as evidence-based. A failure to recognise this in the past has resulted in some of the worst abusive misuses of psychiatric diagnostic concepts. In the final part of the presentation I will outline recent developments in values-based practice in mental health, including some of its applications to diagnostic assessment, and in other areas of health care (such as surgery).