Moral obligations rest on circumstances. But what are these obligating reasons and in virtue of what are they such reasons? Nomological conceptions define such reasons in terms of moral laws. I argue that one such conception cannot be correct and that others do not support the familiar and plausible view that obligating reasons are pro tanto reasons, either because they entail that this view is false or else because they cannot explain—or even help to explain—how it could be true. I also argue that a particular dispositional conception of obligating reasons does support this view of obligating reasons by enabling an explanation of how it could be true. Moreover, my arguments show that the dispositional moral metaphysic on which this conception is predicated can do something that nomological alternatives cannot: explain why obligating reasons and moral obligations are pro tanto reasons and obligations.