People have duties: moral reasons for action that are relatively stringent. At least some of these duties appear to be enforceable. We can forcibly prevent those who would otherwise violate their duties not to kill or inflict harm on innocent, non-threatening people if the force employed is proportionate. It is debatable whether all duties are enforceable. Many would reject the idea that a spouse could be forcibly prevented from violating a duty not to be unfaithful, even if they have committed to a monogamous relationship and therefore have stringent reasons to refrain from infidelity. More controversially, some doubt that duties to assist others who are in severe need are enforceable. What could justify treating some duties as enforceable and others not? In this paper we explore this question.