When, if at all, can a federal political order expel a member state against its will? In political theory, expulsion has—unlike the scenario of secession as voluntary separation—so far received no systematic attention; an omission that is reinforced by the paucity of historical precedent. However, current debates around the potential disintegration of the European Union show that expulsion, as a theoretical and political possibility, deserves a more careful analysis. The article sets out to consider a route toward theorizing expulsion in systematic and realist terms. It outlines possible motivational constellations, feasibility constraints, legitimacy issues, as well as procedural dimensions. The resulting claim—that expulsion may be feasible in some circumstances but that its legitimacy is bound to remain contested—is applied to the case of the European Union.