On those infrequent occasions when the Supreme Court of the United States has addressed limitations that the Constitution imposes on a court's power to choose the law governing resolution of issues arising in multistate litigation, the Court has never reversed a lower court's decision to apply the substantive law of another state. Almost all of the Supreme Court cases treating constitutional limitations on choice of law have involved possible overreaching by courts that applied forum law rather than the conflicting law of another state or nation. This article considers the rare case in which a litigant challenges a state court's decision to apply the substantive law of another state rather than the conflicting law of the forum, a situation that has been described as underreaching by the state court. The conclusion that a court's choice of another state's substantive law might be unconstitutional when that law conflicts with the forum's own substantive law is, at the very least, counter-intuitive.