This textbook aims to explain the principles in international trade theory and show how some useful trade models work. The book concentrates on two fundamental issues in international trade, that is, the "determinants of trade patterns" and the "welfare gains from trade" in various economic environments. Chapters 1 through 3 assume perfect competition and explore the workings of the Ricardian model, the Heckscher-Ohlin-Samuelson model, the Specific Factors model and more recent development of the Eaton-Kortum model. Chapter 4 examines various welfare criteria and their relation to the "social utility function" and, then, proves the basic gains-from-trade proposition. Chapters 5 and 6 examine the implications of imperfect competition using a unified oligopolistic model and variations of the monopolistically competitive model. The roles of the strategic interaction among firms, the economies of scale, product differentiation, the heterogeneity of firms, and the geographic distribution of agents will be highlighted. Chapter 7 deals with some trade policy issues such as the effects of tariffs, the relation of tariffs to other policy measures, and the so-called strategic trade policies.