Moral obligation is a demand of reason—a demanding kind of rational justification. How to understand this rational demand? Much recent philosophy, as in the work of Scanlon, takes obligatoriness to be a reason-giving feature of an action. But the paper argues that moral obligatoriness should instead be understood as a mode of justificatory support—as a distinctive justificatory force of demand. The paper argues that this second model of obligation, the Force model, was central to the natural law tradition in ethics, is truer to everyday intuition about obligation, and also changes our understanding of the problem of moral rationality. A new account is given of why it might be irrational to breach moral obligations. The Force model also sheds new light on moral responsibility, our responsibility for meeting moral obligations. Moral obligation is a standard of reason; but moral responsibility is shown to involve far more than ordinary rational appraisability, precisely because moral obligation involves a distinctive justificatory force of demand—one which specifically governs how we act.