Transnational terrorism poses one of the most serious and growing threats to States and to international peace and security, as shown by the rise of Islamic State of Iraq and the Levant and the phenomenon 'foreign terrorist fighters' from more than 80 countries. The geographically diffuse and complex cellular nature of terrorist networks and infrequent government support seriously undermines international law, in particular the law of jurisdiction, which is one of its most fundamental areas and grounded in the principles of territoriality and nationality. The principles of extraterritorial criminal jurisdiction, since they were first defined by the Harvard Research in International Law in 1935, have yet to be codified in a treaty. As a result, such principles are highly controversial and little understood. This monograph provides, since the Harvard Research, the first comprehensive and empirically informed analysis of the protective principle of extraterritorial criminal jurisdiction. The protective principle permits the State to exercise its national laws over the conduct of persons based abroad that poses a threat to that State's 'vital interests'. Informed by the analysis of State, treaty and institutional practice principally in the terrorism context, the protective principle is, for the first time in recent history, reconsidered, innovatively conceptualised and defined. It provides new insight on how States have used protective principle jurisdiction, as well as some important clarification by identifying and defining 'vital interests' and providing a typology of interests included under the ambit of the protective principle lex lata.