April 22, 2005 Recent evidence indicates that the intensity of economic exchange within and across borders is significantly different: linkages are much tighter within, than among, nation-states. These findings, however, do not necessarily imply that borders and separate national currencies represent significant barriers to trade that should be removed, since the evidence is also consistent with the alternative hypothesis, that domestic exchange is more efficient because domestic producers are better able to satisfy the requirements of local consumers, owing to common tastes and institutions and the existence of local information and social networks. Focusing primarily on trade linkages within and between Canada and the United States, the authors review the evidence on the extent to which national borders lessen the intensity of international economic linkages, primarily trade in goods and services, and the effects on domestic welfare. They also examine the evidence on the impact of common currencies on trade and welfare. They determine that, since the empirical models employed to date in this research cannot distinguish between alternative explanations of the evidence, it is not yet possible to draw firm conclusions for policy-making.