This thesis examines the relationship between international trade institutions and domestic trade policy. Using the example of the GATT and the U.S. tariff schedule, I explain the patterns in protectionism in the early years of GATT negotiations. The research separates theories of protectionism into three categories and attempts to test the relative validity and interaction of the three theory types. First, I examine the impact of international factors, including trading partners, allies and the use of coalitions, on negotiation outcomes. Second, I test the traditional domestic explanations of trade policy setting, including industry size, concentration and potential lobbying effectiveness, as well as the effects of domestic politics and elections on negotiations. The third theory uses the form and function of the international institution in order to predict patterns in U.S. trade policy. These three theories used together, and interacted with one another, provide a well-rounded explanation for the patterns in trade liberalization seen throughout the post-war period in the United States. Using a brand new dataset I collected and coded that covers detailed tariff changes for all goods involved in the U.S. GATT negotiations from 1947 through the end of the Kennedy Round, I test the relative weight of each of the three categories of theory. In addition, I use these new data to provide a thorough description of the development of key industries throughout the early trade rounds.