International law is a set of rules intended to bind states in their relationships with each other. It is largely designed to apply to states, both to constrain (the laws of war) and to empower them (law of sovereignty). Increasingly, international law has been codified, so that today most international obligations are contained in treaty form, although historically customary international law played a relatively more important role than it does today. The role of international law in informing foreign policy decision making has waxed and waned over the course of the past century. It has also varied significantly across countries. It is striking the extent to which international interactions have become reflected in and regulated by formal state-to-state agreements. This article examines international law and international relations, multilateral treaties, political theories of international law (realism, rational/functionalist theories, constructivist approaches), and legal agreements. It also discusses the implementation, compliance, and effectiveness of international law.