Energy and environmental diplomacy factors were a major, though often unrecognized, part of Canada-United States relations prior to the Cold War. Hydroelectricity was the most important of these factors, particularly power exported from the Canadian side of the Great Lakes-St. Lawrence basin. The history of Canada-U.S. hydroelectricity relations, exports, and regulations offered here covers the key steps, institutions, and actors in the evolution of hydroelectricity geopolitics and governance regimes during the first two-thirds of the twentieth century. Most of the earliest large Canadian hydropower projects were built on or planned for border waters, which, after the creation of the 1909 Boundary Waters Treaty, required formal coordination with the United States. Over the following half-century, prospective power projects on border waters continued to dominate the diplomatic dialogue. Federal governments or agencies established broad export restrictions, altering them occasionally, but often subnational governments, special interests, and specific power projects played as large a role in shaping the power trade. The result was a constantly shifting political and regulatory context, albeit one that generally trended toward cooperation and reduced barriers to electricity integration. [ABSTRACT FROM AUTHOR]