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 one of the major conundrums of international criminal law (ICL): how to distinguish international crimes (war crimes, crimes against humanity, terrorism, and genocide) from their domestic analogs (murder, assault, and mayhem). The chapter discusses the various approaches to "internationalization" (focusing on issues of identity, transnationalism, human dignity, global stability, etc.) and concludes that international crimes lack a coherent unifying justification. 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.