Imagine that we are considering whether our healthcare system (or insurer) should fund treatment or procedure X. One factor that may be cited is that of so-called ‘medical necessity’. The claim would be that treatment X should be eligible for funding if it is medically necessary, but ineligible if this does not apply. Similarly, (and relevant to the debates in this special issue), if considering whether a particular treatment should be ethically and/or legally permitted, we may wish to distinguish between cases where the treatment is medically necessary, and those were it is not. But what do we mean by this concept? Here I will propose and briefly defend one plausible and practical definition.