The document summarizes several trade theories, including the Heckscher-Ohlin theory and product cycle model. The Heckscher-Ohlin theory posits that differences in factor endowments, like abundance of labor vs capital, determine comparative advantage and trade patterns. Empirical tests found some inconsistencies, like the Leontief paradox. Later theories incorporated increasing returns to scale, product differentiation, and technological developments over a product's life cycle to better explain trade patterns.