This chapter considers what is potentially encompassed by the term “foreign relations law,” and what it means to think about it as a distinct field of law that can be compared and contrasted across national jurisdictions. The term “foreign relations law” encompasses the domestic law of each nation that governs how that nation interacts with the rest of the world. Many issues of foreign relations law concern allocations of authority between political actors, such as the authority to represent the nation in diplomacy, to conclude and terminate international agreements, to recognize foreign governments and their territories, and to initiate or end the use of military force. But foreign relations law also encompasses issues relating to the role of the courts in transnational cases, such as whether certain issues are “nonjusticiable” and thus subject entirely to political branch determination, whether courts should take into account considerations of international comity when interpreting and applying domestic law, and whether and to what extent courts can apply international law directly to decide a particular case. The chapter describes the historical development of foreign relations law as a field of study within the United States and considers why it has not been treated as a field in many other countries. The chapter concludes by highlighting a central question for foreign relations law, which is the extent to which it should be treated differently than other types of domestic law—referred to in the United States as a debate over “foreign affairs exceptionalism.”