Customary law still appears in many of the decisions of American state and federal courts. Modern courts rely less on customary law, part and parcel of the English common law adopted and adapted by the Founders of the United States, with statutory and administrative law dominating the field. In contrast, the importance of customary law in American Indian tribal courts cannot be understated. Indian tribes now take every measure conceivable to preserve Indigenous cultures and restore lost cultural knowledge and practices. Tribal court litigation, especially litigation involving tribal members and issues arising out of tribal law, often turns on the ancient customs and traditions of the people. But this development of applying customary law in...