Social contract theories hold that political power is ultimately justified by agreement. Putting agreement at the center of politics means that theorists need to show that individuals in a society could agree to the principles that govern them. Many features of social contract theories should be appealing to libertarians, namely their emphasis on individualism and the need for reasoned justification of political power, but despite this, most libertarians have failed to adopt a contractarian justification. In this chapter, I give an overview of contractarianism, explain its appealing features, and argue that it is compatible with libertarianism. Contractarianism not only shows how we might solve some perennial problems in libertarian thought, it provides libertarianism with distinctively political justification that can appeal to many other points of view.