This paper first considers the policy reasons for why the international community should define terrorism, focusing on arguments that terrorism:(a) seriously violates human rights;(b) jeopardizes the State, deliberative politics and the constitutional order which sustains rights;(c) is politically or publicly motivated violence distinguishable from private violence;(d) threatens international peace and security; and (e) requires definition to control the operation of mandatory Security Council measures since 2001, which have empowered States to unilaterally define and criminalize terrorism to suit their own sovereign interests. Secondly, this paper briefly outlines recent proposals for an international definition of terrorism before extrapolating the basic elements of an international definition of terrorism from the policy reasons for definition discussed in the first part of this chapter. Finally, claims that certain conduct should be excluded from any definition of terrorism are considered. A coherent legal definition of terrorism might help to confine the misuse of the term by national governments against their political opponents and in ways which seriously undermine fundamental human rights.