This work is a chapter for a forthcoming book on The Essentials of International Criminal Law to be published by Aspen Publishers as part of its Essentials series. This chapter discusses the contested concept of terrorism under international law, with a focus on efforts to criminalize the phenomenon. In particular, it discusses the major obstacles to achieving an omnibus definition of the crime with a particular emphasis on the intersection of the prohibitions against terrorism and international humanitarian law. Additional chapters in the text address the history of ICL, the sources of ICL, the major international crimes and defenses, and ICL reasoning and rhetoric. Publication is expected in October 2008.