This chapter explains what liberalism is. It is easy to list famous liberals, but it is harder to say what they have in common. John Locke, Adam Smith, Montesquieu, Thomas Jefferson, John Stuart Mill, Lord Acton, T. H. Green, John Dewey, and contemporaries such as Isaiah Berlin and John Rawls are certainly liberals. However, they do not agree on issues such as the boundaries of toleration, the legitimacy of the welfare state, and the virtues of democracy. They do not even agree on the nature of the liberty they think liberals ought to seek. The chapter considers classical versus modern liberalism, the divide within liberal theory between liberalism and libertarianism, and liberal opposition to absolutism, religious authority, and capitalism. It also discusses liberalism as a theory for the individual, society, and the state.