US President Donald Trump has said he 'will never, ever apologize for protecting the safety and security of the American people.' But what is security and how does US security policy relate to ideas about American grand strategy and geopolitics? This module engages critically with the centrality of 'security' in US foreign and defense policy by drawing on insights from the study of international security. Particular attention is given to the role of security in determinations of American grand strategy and the subsequent policy outcomes and developments in a number of security areas. After an analysis of traditional US concerns over defense and military security, we will turn the bulk of our attention to how the United States has confronted so-called new security challenges such as homeland security, counterterrorism, cybersecurity, environmental security, human security, and the role of identity and culture in US national security. The module concludes by pointing to the future opportunities, challenges, and constraints the US faces in the governance of international security.