This article offers a definition of suicide, consisting of five individually necessary & jointly sufficient conditions. The definition is then expanded upon at some length, especially in relation to the crucial notions of intention & intentional action. Standard cases of suicide & nonsuicide are used to test the definition for adequacy & explanatory power. A number of controversial cases (those of Bobby Sands, Socrates, & Captain Oates) & kinds of cases (self-sacrifice, martyrdom, & refusal of treatment) are examined in light of it.