This paper reviews the scientific contributions of Paul Krugman to the study of international trade, on the occasion of his receipt of the 2008 Nobel Memorial Prize in Economics. A simplified exposition is presented of some of his principal findings, including: the effects of trade on firm scale and product diversity in a general model of monopolistic competition; the integration of monopolistic competition with factor endowments theory; the implications of transport costs, including home-market effects and the possibility of agglomeration in models of economic geography; and the positive and normative consequences of oligopolistic trade.