From its earliest decisions in the 1790s, the US Supreme Court has used international law to help resolve major legal controversies. This book presents a comprehensive account of the Supreme Court’s use of international law from its inception to the present day. Addressing treaties, the direct application of customary international law and the use of international law as an interpretive tool, this book examines all the cases or lines of cases in which international law has played a material role, showing how the Court’s treatment of international law both changed and remained consistent over the period. Although there was substantial continuity in the Supreme Court’s international law doctrine through the end of the nineteenth century, the past century has been a time of tremendous doctrinal change. Few aspects of the Court’s international law doctrine remain the same in the twenty-first century as they were two hundred years ago.