This article examines the most pertinent questions relating to the applicability of the right of self-defense to attacks conducted by non-State armed groups (NSAGs) acting independently of State control from the territory of one or more States against the territory of another State. These questions are approached from the perspective of legality (does the right of self-defense apply to attacks not mounted by or under the control of a State) and modality (assuming the applicability of self-defense to such attacks; how do the principles of necessity, proportionality and immediacy affect its application)? Starting with an assessment of the place of self-defense in international law at the time the U.N. Charter was adopted, it proceeds with an examination of State practice before and after the 9/11 attacks. The 9/11 attacks triggered not only increased reliance upon self-defense in relation to attacks by NSAGs, but also an ongoing debate to which this article is intended as a constructive contribution. After concluding that there is substantial and increasing, albeit not universal support for the applicability of self-defense to attacks by NSAGs, the modality of its application is discussed. In that context, the principle of necessity in the context of self-defense is presented as being of paramount importance in answering the question of under which circumstances self-defense against NSAGs can be exercised on the territory of another State and how such action relates to the rights of the State where the NSAG is located and conducting operations from.