Although the president and Congress often set the agenda for foreign policymaking, the day-to-day conduct and administration of U.S. foreign policy are vested in the massive federal bureaucracy. The U.S. foreign policy bureaucracy is roughly divided between four complexes: diplomatic, security, intelligence, and economic. Each of these issue areas contains multiple actors who, despite having similar goal, are constantly vying for access to resources for their organization. Studying the foreign policy bureaucracy is critical to understanding the foreign policy process. The structures and agencies in the federal system outlast members of Congress and presidents, who have limited time, resources, and capacities that prevent them from engaging in day-to-day and even long-term foreign policy issues. Bureaucrats are vested with the resources, budgets, and expertise to implement policies over the long term and provide much-needed “continuity and constancy” to foreign policymaking. Each of the four complexes, however, faces challenges and constraints on its mission. Many of these obstacles arise from cultural and institutional factors within the agency and the bureaucracy. Overall, the U.S. foreign policy bureaucracy is highly fragmented and decentralized, despite Congress’s efforts to centralize the foreign policy decision-making process. This conflict is yet another paradox of U.S. foreign policy.