Diagnoses are central to the practice of medicine, where they serve a variety of functions for clinicians, patients, and society. They aid communication, explain symptoms, inform predictions, guide therapeutic interventions, legitimize sickness, and authorize access to resources. Insofar as psychiatry is a discipline whose practice is shaped by medical conventions, its diagnoses are sometimes presented as if they serve the same sorts of function as diagnoses in bodily medicine. However, there are philosophical problems that cast doubt on whether the functions of psychiatric diagnoses can legitimately be considered to be equivalent to those of medical diagnoses. The aim of this chapter is to explicate some of these problems, particularly conceptual and epistemological problems pertaining to the roles of diagnoses as explanations. I begin with an overview of the various functions that medical diagnoses normally serve and suggest that many of these functions receive justificatory support from the explanatory roles of the diagnoses. I then present issues regarding the epistemic functions of psychiatric diagnoses and how these issues have featured in the arguments of prominent critics of psychiatry