This book is an accessible and innovative introduction to the key issues and emerging challenges of US national security policy in the twenty-first century. The text explores how the United States, as the hegemonic player in the international arena, defines its security interests and objectives, perceives and articulates threats to these goals, and develops polices to protect and enhance its security objectives to counter these threats. Going beyond existing works, it merges insights from the disciplines of US foreign policy analysis, US defence studies, and international security studies, and thereby provides a more holistic understanding of America's quest for security in an age of uncertainty. The book is divided into two sections. The first provides the background - the analytical 'toolbox' - to investigate America's approaches to security in the post-Cold War and post-9/11 eras. This section introduces the reader to the idea of 'security', and what the key concepts are in the study of international security. It also explores relevant theoretical perspectives, actors, institutions, and processes important to understand the making of US security policy, and gives a short overview of the institutional and ideological origins of post-Cold War US security policymaking. Part II examines US approaches to security from the end of the Cold War to the present day, including the administrations of George H.W. Bush, William J. Clinton, George W. Bush, and Barack Obama, divided into three phases: the post-Cold War 'interregnum', the post-9/11 period, and the contemporary era. Each chapter provides an overview of key policies, actors, and process during each specific phase, as well relating the US approach to security to both traditional and non-traditional issues conceptualized in the study of international security. The book concludes by recapitulating the US approach to security and by pointing to the future opportunities, challenges, and constraints the US faces in the governance of international security in an age of uncertainty. This textbook will be a valuable guide for students of US national security, US foreign policy, US defence policy, and international security and IR in general.