2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/230013This note provides a summary of the meaning of the term `Superselection Rule' in Quantum Mechanics and Quantum-Field Theory. It is a slightly extended version of a contribution to the Compendium of Quantum Physics: Concepts, Experiments, History and Philosophy, edited by Friedel Weinert, Klaus Hentschel, Daniel Greenberger, and Brigitte Falkenburg.10 pages, no figures. Two easy-to-read proofs of statements made in the text are added in the second version, with slight adaptations in notationQuantum Physicstext