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 history of international criminal law (ICL), drawing upon major developments in the law of armed conflict, international human rights law, and the criminal prohibitions against piracy and the slave trade. The chapter interweaves the history of substantive norms with that of evolving principles of domestic and international jurisdiction, as these narratives are virtually inseparable in ICL. Additional chapters in the text will address the sources of ICL, the major international crimes and defenses, and ICL reasoning and rhetoric. Publication is expected in 2007 for adoption in 2008.