A complete theory of harming must have both a substantive component and a formal component. The substantive component, which Victor Tadros (2014) calls the “currency” of harm, tells us what I interfere with when I harm you. The formal component, which Tadros calls the “measure” of harm, tells us how the harm to you is related to my action. In this chapter I survey the literature on both the currency and the measure of harm. I argue that the currency of harm is well-being and that the measure of harming is best captured by a causal account on which harming is causing a harm. A harm for you is the presence of something intrinsically bad for you or the absence of something intrinsically good for you. Thus, although a counterfactual account of the measure of harm need not distinguish between an harm and a harmful event, the causal account reserves the term ‘harm’, not for a harmful event, but only for its effect. Finally, I show how a complete theory of harming can help us to answer questions about whether we can harm people with speech, whether we can harm the dead, and how it is possible to harm future generations.