This is a chapter in the second edition of The National Security Enterprise, a book edited by Roger Z. George and Harvey Rishikof that provides practitioners' insights into the operation, missions, and organizational cultures of the principal national security agencies and other institutions that shape the U.S. national security decision-making process. Unlike some textbooks on American foreign policy, it offers analysis from insiders who have worked at the National Security Council, the State and Defense Departments, the intelligence community, and the other critical government entities. The book explains how organizational missions and cultures create the labyrinth in which a coherent national security policy must be fashioned. Understanding and appreciating these organizations and their cultures is essential for formulating and implementing it. Taking into account the changes introduced by the Obama administration, the second edition includes four new or entirely revised chapters (Congress, Department of Homeland Security, Treasury, and USAID) and updates to the text throughout. It covers changes instituted since the first edition was published in 2011, implications of the government campaign to prosecute leaks, and lessons learned from more than a decade of war in Afghanistan and Iraq. This up-to-date book will appeal to students of U.S. national security and foreign policy as well as career policymakers.