This chapter explains the harm principle and defends the nature of harm as it is used in that principle. Offering an account of liberal theory—or any account of political morality—requires offering normative principles. The chapter introduces John Stuart Mill's harm principle in order to situate toleration in liberal moral theory. It discusses some preliminary distinctions that will be useful later and introduces Joel Feinberg's influential account of harm. The chapter looks at Feinberg's account of harm as an event-based account of harm rather than a state-based account. The harm principle indicates when it is permissible to interfere with an individual and does so with reference to harm. The chapter provides a conceptual analysis of harm, as that term is used in the harm principle. Much of the contemporary debate about the nature of harm, in particular whether it is state-based or event-based, comes down to determining which sort of account best handles possible cases.