Book excerpt: The debate over the use of foreign law on the U.S. Supreme Court is a part of a much larger constitutional conversation occurring in many jurisdictions regarding the migration of constitutional ideas. The migration of constitutional ideas across legal systems is rapidly emerging as one of the central features of contemporary constitutional practice. The migration of constitutional ideas has been identified to a limited extent at a descriptive level. But many basic conceptual issues have received almost no attention in the large and growing critical literature on comparative constitutional law. For example, the existing literature has not addressed systematically the methodology of constitutional migration, nor the normative underpinnings of this enterprise. The Migration of Constitutional Ideas tackles this phenomenon from a diverse range of methodological perspectives. In this introductory essay, I describe and defend a theory of the use of comparative constitutional materials in constitutional interpretation which I term dialogical interpretation. The goal is to use comparative materials as an interpretive foil, to expose the factual and normative assumptions underlying the court's own constitutional order. First, comparative materials are engaged to identify the assumptions embedded in positive legal materials. But in the process of articulating the assumptions underlying foreign jurisprudence, a court will inevitably uncover its own. The next move is to engage in a process of justification. If the assumptions are different, the question becomes why they are different. Comparative engagement highlights the contingency of legal and constitutional order, and opens for discussion and contestation those characteristics which had remained invisible to whether those assumptions ought to be shared. Finally, the court is faced with a set of interpretive choices. A court may be able to justify the similarity with, or the difference between, the assumptions underlying its own constitutional order and a foreign one. Comparative engagement, then, leads to a heightened sense of legal awareness through interpretive clarification and confrontation. But the identification and attempted justification of constitutional assumptions through comparison may lead a court to challenge and reject those assumptions and search for new ones. In cases of constitutional similarity, a court may determine a difference to be unfounded, and may rely on comparative jurisprudence as the engine of legal change.