In this chapter, our focus is the role played by notions of rationality in the diagnosis of mental disorders, and in the practice of overriding patient autonomy in psychiatry. We describe and evaluate different hypotheses concerning the relationship between rationality and diagnosis, raising questions about what features underpin psychiatric categories. These questions reinforce widely held concerns about the use of diagnosis as a justification for overriding autonomy, which have motivated a shift to mental incapacity as an alternative justification. However, this (...) approach too has recently been criticized from a mental disability rights perspective. Our analysis of the relationship between mental capacity and rationality is used to illuminate these concerns, and to investigate further the relationship between rationality and psychiatric diagnosis. (shrink)