This report provides an introduction to the roles that international law and agreements play in the United States. International law is derived from two primary sources-international agreements and customary practice. Under the U.S. legal system, international agreements can be entered into by means of a treaty or an executive agreement. The Constitution allocates primary responsibility for entering into such agreements to the executive branch, but Congress also plays an essential role. First, in order for a treaty (but not an executive agreement) to become binding upon the United States, the Senate must provide its advice and consent to treaty ratification by a two-thirds majority. Secondly, Congress may authorize congressional-executive agreements. Thirdly, many treaties and executive agreements are not self-executing, meaning that implementing legislation is required to provide U.S. bodies with the domestic legal authority necessary to enforce and comply with an international agreement's provisions. The status of an international agreement within the United States depends on a variety of factors. Self-executing treaties have a status equal to federal statute, superior to U.S. state law, and inferior to the Constitution. Depending upon the nature of executive agreements, they may or may not have a status equal to federal statute. In any case, self-executing executive agreements have a status that is superior to U.S. state law and inferior to the Constitution. Treaties or executive agreements that are not self-executing have been understood by the courts to have limited status domestically; rather, the legislation or regulations implementing these agreements are controlling. The effects of the second source of international law, customary international practice, upon the United States are more ambiguous and controversial. While there is some Supreme Court jurisprudence finding that customary international law is part of U.S. law, U.S. statutes that conflict with customary rules remain controlling. Customary international law is perhaps most clearly recognized under U.S. law via the Alien Tort Statute (ATS), which establishes federal court jurisdiction over tort claims brought by aliens for violations of "the law of nations." Recently, there has been some controversy concerning references made by U.S. courts to foreign laws or jurisprudence when interpreting domestic statutes or constitutional requirements. Historically, U.S. courts have on occasion looked to foreign jurisprudence for persuasive value, particularly when the interpretation of an international agreement is at issue, but foreign jurisprudence never appears to have been treated as binding. Though U.S. courts will likely continue to refer to foreign jurisprudence, where, when, and how significantly they will rely upon it is difficult to predict.