This chapter defines transnational crime and other key concepts. It first defines transnational crime and transnational criminal organizations (TCOs). It covers the rise of transnational crime with a focus on TCOs, their ‘new’ (networks) and ‘old’ (markets and hierarchies) organizational forms, and other useful concepts in the study of transnational crime. Second, it discusses the crime–terror debates, as criminal groups often utilize political violence for their profit-seeking ends, and terror groups use criminal activities for fundraising. It then moves on to explore the relationship between transnational organized crime (TOC) and the state, and the implications for global and national security of states. It provides a case study of TCOs in Mexico that illustrates many of the trends in organized crime since the end of the Cold War to the present. Finally, the chapter addresses how the international community has combatted transnational criminal actors as they become more powerful and increasingly diversify their revenue streams.