The book provides an approach that is more comprehensive than other contributions within this field. It is located at the intersection of two central and challenging issues in international law. The first concerns the ways in which such normative frameworks change, evolve or are modified in international law. The second concerns the extent to which the basic norms governing the use of force against terrorist have changed significantly since the attacks on New York and Washington DC, in 2001. The international rules governing the use of force in international relations have been under pressure in recent years. They have on several occasions been challenged by states' practice, be it through actual acts (for example in the case of Kosovo 1999, Afghanistan 2001 and Iraq 2003) or in statements (such as the 2002 US National Security Strategy). A fundamental question concerns how international law reacts to such challenges: does is disintegrate or does it adapt to the new circumstances? It is found that international law in this area has in fact developed and adapted to meet new challenges posed by terrorism.