This study provides a timely and useful benchmark for analysis of the effects of the recently negotiated North American Free Trade Agreement on investment flows. It also presents a unified history of foreign investment in Canada, Mexico, and the United States over the twentieth century, stressing interactions among these countries and their changing policies towards inward and outward investment. Twomey analyzes economic theories of foreign investment from the perspectives of neoclassical economics and political science and places them in the context of the ongoing debate over neo-protectionist policies and the role of the United States in the global economy.