Governments claim to have to possess two unique moral powers. “Legitimacy” refers to a supposed permission to create rules, and enforce those rules using coercion and violence, over a certain people in a geographic area. “Authority” refers to a supposed moral power to create in others an obligation or new ground of obligation to obey these rules. This chapter clarifies what these concepts mean, and then examines how different libertarians and classical liberals have argued for, or against, government legitimacy and authority, in particular, examining debates about whether governments should exist, what the scope of their power should be, whether it is permissible to resist injustice, and why.