This work is an introductory chapter for a forthcoming book on Understanding International Criminal Law to be published by Aspen Publishers as part of Aspen's Essentials series. This chapter presents a succinct definition of international criminal law (ICL), situating it within the fields of public international law, transnational law, international human rights, and international humanitarian law. Additional chapters in the text will address the history of ICL, the sources of ICL, the major international crimes and defenses, and ICL reasoning and rhetoric. Publication is expected in 2007 for adoption in 2008.