According to the Harm Principle, roughly, the state may coerce a person only if it can thereby prevent harm to others. Clearly, this principle depends crucially on what we understand by harm. Thus, if any sort of negative effect on a person may count as a harm, the Harm Principle will fail to sufficiently protect individual liberty. Therefore, a more subtle concept of harm is needed. I consider various possible conceptions and argue that none gives rise to a plausible version of the Harm Principle. Whether we focus on welfare, quantities of welfare or qualities of welfare, we do not arrive at a plausible version of this principle. Instead, the concept of harm may be moralized. I consider various ways this may be done as well as possible rationales for the resulting versions of the Harm Principle. Again, no plausible version of the principle turns up. I also consider the prospect of including the Harm Principle in a decision-procedure rather than in a criterion of rightness. Finally, in light of my negative appraisal, I briefly discuss why this principle has seemed so appealing to liberals.