The rule of law, once widely embraced and emulated, now faces serious threats to its viability. To get our bearings we must return to first principles. This book articulates and defends a comprehensive, coherent, and compelling conception of the rule of law and defends it against serious challenges to its intelligibility, relevance, and normative force. The rule of law's ambition, it argues, is to provide protection and recourse against the arbitrary exercise of power using the distinctive tools of the law. Law provides a bulwark of protection, a bridle on the powerful, and a bond constituting and holding together the polity and giving public expression to an ideal mode of association. Two principles immediately follow from this core: sovereignty of law, demanding that those who exercise ruling power govern with law and that law governs them, and equality in the eyes of the law, demanding that law's protection extend to all bound by it. Animating law's rule, the ethos of fidelity commits all members of the political community, officials and lay members alike, to take responsibility for holding each other accountable under the law. Part I articulates this conception and locates its moral foundation in a commitment to common membership of each person, recognizing their freedom, dignity, and status as peers. Part II addresses serious challenges currently facing law's rule: finding a place in the legal system for equity, mercy, and effective responses to emergencies, taming the new leviathans of the digital world, and extending law's rule beyond national borders.