Obligations are requirements and requirements are necessities. Naturally, then, obligations are necessities. This compelling syllogism provides an important clue to the logic of obligation: it’s simply the logic of necessity, suitably restricted.Fortunately, the logic of necessity is well understood. If we interpret the standard notation of modal logic in the appropriate way, letting box abbreviate ‘It ought to be that’ and diamond ‘It is permissible that’, then we can fiddle around with our Kripke models to formulate a complementary semantic theory (Montague 1960). Incorporating familiar tools for the representation of context sensitivity – specifically, a modal base and an ordering source – enables the logic of necessity to serve as a semantic theory for a rich and philosophically interesting fragment of natural language (Kratzer 2012). The precision and uniformity of this approach, coupled with its empirical successes over several decades of fruitful research, offers a powerful reason to think that little more is required. Perhaps we should expect some fine-tuning here and there, but future revisions will preserve the foundational assumption that the grammar and meaning of ‘ought’ is a special case of the grammar and meaning of box. Portner (2009: 47) and Chrisman (2016: 69–73) call this the standard theory of deontic modals.