Recent years have seen increasing reliance on self-defence as a concept justifying the use of force against armed attacks by terrorists operating from within a foreign state. While the matter has prompted major debates, a good case can be made that contemporary international law now begins to recognise a right of self-defence against attacks by terrorists. (We will refer to this in the following as ‘anti-terrorist self-defence’, meaning a response directed against an attack carried out by a terrorist organisation.) This article deliberately does not rehearse the major debates about the scope of self-defence, but moves beyond them. It seeks to assess whether and how the two main limitations on the exercise of self-defence – namely necessity and proportionality – could be adapted to self-defence outside the inter-state setting. It shows that both limitations do indeed require some adaptation in order to be applied to anti-terrorist self-defence. With respect to necessity, we argue that this adaptation is actually a chance to make sense of a requirement that is of very limited practical relevance in the inter-state setting. As the article shows, necessity can assume a crucial role in the anti-terrorist setting: in order for trans-border uses of force to be necessary, states invoking self-defence must demonstrate that the terrorist threat cannot be repelled by the host state (on whose territory the terrorists are based). As for proportionality, recent debates about anti-terrorist self-defence expose the weaknesses of proportionality as a restraint on self-defence. Construed primarily as a prohibition against excessive force, even in the inter-state setting, proportionality has always been dependent on the nature of the aims pursued in the name of self-defence. Recent anti-terrorist practice suggests that many states are prepared to accept far-reaching uses of force against terrorists. It also suggests that, in applying proportionality, the targeted (or otherwise) character of the response is of crucial importance. The law is clearly in a state of flux, but in debates such as those about Turkey's use of force against the PKK or Israel's military operations against Hamas or Hezbollah, one can see the beginning of a process of developing guidelines on proportionality. © 2012, Cambridge University Press and The Faculty of Law, The Hebrew University of Jerusalem. All rights reserved.