The document summarizes theories of international trade, including the Heckscher-Ohlin theory and its assumptions. The Heckscher-Ohlin theory states that differences in factor endowments between countries determine comparative advantage and trade patterns. It includes the Heckscher-Ohlin theorem and factor price equalization theorem. Empirical tests by Leontief found results contrary to predictions, known as the Leontief paradox. The document also discusses economies of scale, intra-industry trade, technology gaps, and product cycle models as additional determinants of trade patterns.