There have been serious tensions in the Canada-United States relationship in recent years, sparked by both a marked divergence in the domestic and international policy priorities in the two countries, particularly under the Bush and Chretien administrations, and an obvious lack of rapport between the two leaders. In the sphere, however, the Canada-U.S. relationship has been minimally affected by such friction. In fact, it is argued here that the environmental interoperability of the two countries has recently broadened and deepened, and this can be traced to three developments. First, there is a high degree of transgovernmentalism, or effective mid-level working relations among officials on both sides of the border that has become entrenched over decades of cooperation on various issues. Moreover, cross-border scientific and technical relationships between Canadians and Americans have been solidified through the implementation of existing agreements and a burgeoning system of continental-level professional interactions. Finally, there is a thickening network of relationships at the subnational and cross-border regional level that is changing the very nature of Canada-U.S. relations. These subnational and regional relationships are driven more by pragmatic concerns about economic interdependence and shared ecosystems, even a sense of bioregionalism, than by national-level politics. While all of these factors contribute to a de-politicization of the Canada-U.S. relationship more generally, national political dynamics do have an indirect impact on interoperability. First, federal electoral outcomes and the ideology of the governing party in both countries has an impact on the priority placed on protection as well as the approach to bilateral relations. Second, cutbacks to federal programs in the U.S and severe budgetary constraints in American states certainly pose challenges for cross-border initiatives. Even so, funding for cooperation in areas such as air quality and climate change has continued, with the Canadian federal government playing a key role in helping to finance new initiatives. In fact, the current minority government situation in Canada is serving to encourage expenditures. Finally, the simmering tension between the Bush administration and states that have been testing the limits of their and transboundary decision-making powers is likely to come to a head, with potential repercussions for some cross-border regional initiatives. Environmental Interoperability in the Canada-U.S. Relationship There can be little doubt that the Canadian-American bilateral relationship has gone through a rough patch over the past few years. Leaving aside the much publicized diplomatic slights, we have seen a marked divergence in international and domestic policy priorities in the two countries, particularly under the Bush and Chretien administrations after September 11. The Bush administration's active support for unilateralism and pre-emption abroad does not sit well with Canada's support for multilateralism, leadership by international institutions, and a focus on human security. Even the philosophical underpinnings of domestic policies in the two countries have taken different paths. Neither country is comfortable with the other's approach to drugs, gay rights, gun control, or abortion, and policies in the area of health care and social supports seem, if anything, to be diverging. Although some of these policy differences are enduring ones, others bear the imprimatur of Bush and Chretien, who certainly did not see eye to eye on most issues. While Prime Minister Martin has made a concerted attempt to personally connect with President Bush, any rapport between the two individual leaders will continue to be strained by such policy differences. Yet, such tensions and differences have had, and are likely to have, little impact on the Canadian-American relationship. …