This chapter provides an overview of how globalization has changed what security is and how security can be achieved most effectively. The first section provides a brief overview of the literature on globalization and security, highlighting a disconnect between the state-centric security literature and the focus on transnational linkages within the literature on globalization. The second section shows the importance of accounting for the effects of globalization when studying security. Globalization has made the threat environment more diverse and complicated for understanding the dynamics of the states system. The third section addresses the interaction of core issues at the domestic, international, and transnational levels highlighting how these interactions challenge or reinforce the traditional basis of legitimacy and authority of states and the state system. The final section concludes with a discussion of implications of the impact of globalization on security.