This paper states that individuals' duties have a considerably more extensive role in contemporary constitutionalism than usually attributed to them. While explicit constitutional duties are not the general rule, several scholarly and judicial constructs are centered on duty-based considerations. Such is the case of the doctrine of third party effect of fundamental rights, the thesis of correlativity of rights and duties, and some conceptions regarding limitations of fundamental rights. That being the case, it discusses possible justifications for the express constitutionalization of duties.