This chapter examines the threats posed by transnational crime to national security. Globalization and other international trends may have the unintended consequence of fostering the development of transnational crime. Initial state and international responses to transnational crime in the 1980s were driven in large part by the U.S. war on drugs. After providing an overview of relevant definitions and key concepts, particularly with respect to international crime and organized crime, the chapter considers both the reasons for and the nature of the increase in transnational crime since the end of the Cold War. It then looks at debates over the strength and nature of the ‘nexus’ between transnational crime and terrorism. It concludes by analysing how the government response to transnational crime has evolved over time, focusing on increased coordination and securitization between nations.