This chapter contrasts two ways in which terrorism might be defined for legal purposes. The first treats terrorism as a “binary concept”: a single set of necessary and sufficient conditions that an agent or an act must meet in order to count as terroristic. A potential problem with this approach is that it fails to distinguish different sorts of terrorist groups in the way we might want our legal definition of terrorism to do. A more promising approach, in which terrorism is treated as a “family-resemblance concept”—a set of distinct criteria, not all of which need to be present for a genuine case of terrorism to arise—is outlined. Finally, it is considered whether recent changes in the nature of terrorism should affect how we characterise terrorism in law.