There is much controversy concerning the role of international and foreign legal sources in U.S. courts. Although in some contexts this subject can seem abstract, it becomes very concrete when the international source involves “customary international law,” which is considered to be part of the Law of Nations. Customary international law can provide the basis for a federal cause of action where Congress has not created one. Understanding why and  how this occurs requires analysis of the U.S. Constitution, federal statutes, and a series of key decisions, most notably the 2004 U.S. Supreme Court decision in Sosa v. Alvarez-Machain. This article will present the essential framework of analysis and identify the key open issues....