The self-determination of peoples is a fundamental legitimating principle of the international system; it justifies the system’s very existence. Through a vast diachronic corpus and pertinent data sets, this article nevertheless reveals a puzzling decline in the public discourse on, and practice of, self-determination over the last 50 years. I identify and assess four structural explanations for this decline: "lexical change" (replacing self-determination with alternative terms); "silent hegemony" (taking the norm for granted); "reactive rhetoric" (echoing conflicts and new state formation post hoc); and "mission accomplished" (rectifying the incongruence between national boundaries and state borders). Complementing these structural causes with agential reasons, I further suggest that powerful state actors and persuasive academics have sought to "tame" self-determination as both principle and practice, retaining the term but altering its meaning from a source of threat into a resource for containing it. Self-determination, however, has not been eliminated, and taming it may yet prove a pyrrhic victory.