The concept of medical necessity is often used to explain or justify certain decisions—for example, which treatments should be allowed under certain conditions—as though it had an obvious, agreed-upon meaning as well as an inherent normative force. In introducing this special issue of Clinical Ethics on medical necessity, we argue that the term, as used in various discourses, generally lacks a definition that is clear, non-circular, conceptually plausible, and fit for purpose. We propose that future work on this concept should (...) address three main questions: what medical necessity is (i.e., what makes something medically necessary, as opposed to something else); what the concept does (what ‘work’ is it doing when invoked in different settings); and what should follow, normatively, from the fact that something is indeed medically necessary (on some plausible conception). (shrink)