Diplomacy’s role in foreign policy is hampered by multiple understandings of what diplomacy is and does. A broad definition of diplomacy holds that it encompasses more than the promotion of peaceful international relations. Instead, it applies to the sum of those relations—peaceful, hostile, and everything in between. Thus, foreign relations—so long as they involve the interests, direction, and actions of a sovereign power—may be regarded as being synonymous with diplomatic relations, whereby foreign policy relates to the theory and practice of setting diplomatic priorities; planning for contingencies; advancing strategic, operational, and tactical diplomatic aims; and adjusting those aims to domestic and foreign constraints. This conception of diplomacy is functional: it emphasizes the roles of diplomats and recognizes that many other people perform these roles besides official envoys; and it illustrates that diplomatic settings—and the means, methods, and tools of diplomacy—undergo continuous change. The basic mediating purpose of diplomacy, however, has endured, as has much of its institutional apparatus—embassies, ambassadors, treaties, and so on. This is likely to remain the case so long as there are multiple polities in the world, all having to relate to one another.