With the waves of reform occurring in mental health legislation in England and other jurisdictions, mental capacity is set to become a key medico-legal concept. The concept is central to the law of informed consent and is closely aligned to the philosophical concept of autonomy. It is also closely related to mental disorder. This paper explores the interdisciplinary terrain where mental capacity is located. Our aim is to identify core dilemmas and to suggest pathways for future interdisciplinary research. The terrain (...) can be separated into three types of discussion: philosophical, legal and psychiatric. Each discussion approaches mental capacity and judgmental autonomy from a different perspective yet each discussion struggles over two key dilemmas: whether mental capacity and autonomy is/should be a moral or a psychological notion and whether rationality is the key constitutive factor. We suggest that further theoretical work will have to be interdisciplinary and that this work offers an opportunity for the law to enrich its interpretation of mental capacity, for psychiatry to clarify the normative elements latent in its concepts and for philosophy to advance understanding of autonomy through the study of decisional dysfunction. The new pressures on medical and legal practice to be more explicit about mental capacity make this work a priority. (shrink)