Efficient constraint propagation is crucial to any constraint solver. We show that watched literals, already a great success in the satisfiability community, can be used to provide highly efficient implementations of constraint propagators. We describe three important aspects of watched literals as we apply them to constraints, and how they are implemented in the Minion constraint solver. We show three successful applications of to constraint propagators: the sum of Boolean variables; GAC for the ‘element' constraint; and GAC for the ‘table' constraint.