This chapter begins by outlining trends in comparative citation, when courts look to foreign courts’ rulings in making their own decisions. Such trends include borrowing in areas of constitutional structure, constitutional interpretation techniques and modes of analysis, and comparative jurisprudence. Courts attach varying weights to such precedent: some considering it mandatory, others advisable, and yet others as voluntary. The author notes a declining American hegemony in comparative citation and the ascendency of new jurisprudential authority in the area (e.g. Canada, Germany, the European Court of Human Rights). The chapter outlines explanations from history, public law, and social science for where, when, and how constitutional courts and judges engage in comparative reference, and identifies significant reasons for comparative citations such as necessity, a desire to effect change within one’s national constitutional regime, judicial quest for legitimacy and well-grounded reasoning, and appeals to external authority in fledgling democratic systems. It concludes by exploring epistemological and methodological challenges of comparative citation research such as a reluctance to admit its historical roots, its focus on the Western liberal-democratic constitutional settings, the selective deployment of reference to foreign precedent, and the difficulty of tracking implicit borrowing.