Constitutional comparativism—the notion that international and foreign material should be used to interpret the U.S. Constitution—is gaining currency. Yet proponents of this practice rarely offer a firm theoretical justification for the practice. This Article contends that constitutional comparativism should be examined from the perspective of constitutional theory. The use of comparative and international material must be deemed appropriate or inappropriate based on a particular judge’s interpretive mode of constitutional analysis. The Article presents four classic constitutional theories—originalism, natural law, majoritarianism, and pragmatism—and addresses the propriety of constitutional comparativism under each theory. This theoretical approach goes far to explain why particular judges embrace comparativism, while others eschew it. In so doing, it grounds the debate in the larger framework of classic constitutional theory. It also anticipates the disquiet that constitutional comparativists will experience at the inadequacy of any existing constitutional theory to capture fully the comparative agenda. It therefore introduces the broad outlines of a comparative constitutional theory and judges such a theory based on established criteria for its saliency.