Daniel Bethlehem’s note on self-defense principles is intended to stimulate debate on one of the most contentious issues facing the international community today, namely, the legal response to imminent or actual terrorist attacks by nonstate actors. The note contains a set of principles that are sensitive to the practical realities of the circumstances that it addresses. But it is also intended to take up a legal policy matter—to create or amend principles of international law related to the use of armed force in dealing with threats from nonstate actors. To create or amend these principles, there must be clear evidence and sufficient state practice, or at least opinio juris, pointing toward the change of existing rules or the creation of new rules to “fill the gap.” The whole balance in international law among the various rights, obligations, and interests of international actors will be compromised if the notion of self-defense is to be expanded beyond its legitimate limitations. As illustrated below by some basic examples drawn from the existing law of self-defense, there is sufficient flexibility in the current legal order to allow for the lawful exercise of self-defense in response to most situations of armed terrorist attacks.