This thesis consists of three independent papers, ordered chronologically with respect to when they were initiated. Empirical research has established that there are large and persistent productivity differences among firms in narrowly defined industries (Bartelsman and Doms, 2000). Other studies, in particular Bernard and Jensen (1999), have shown the existence of a causal link running from ex-ante firm productivity to export decisions. Furthermore, exposure to trade has been found to enhance growth opportunities only for some firms, reallocating market shares and resources toward the more productive ones and contributing thus to aggregate productivity growth (Clerides, Lach and Tybout, 1998; Bernard and Jensen, 2004). These findings have led to the development of new theoretical models emphasizing the interaction between firm heterogeneity and fixed market entry costs in generating international trade and inducing aggregate productivity growth. The first and third chapters of this thesis extend the framework developed by Melitz (2003) to analyze the implications of firm heterogeneity for old and new issues in international trade. The first paper studies the effect of trade liberalization between countries that differ in their relative endowment of skilled workers when growth-promoting R&D activities are skill intensive with respect to goods production. In particular, the analysis focuses on the changes that falling trade costs induce on consumer welfare and on the number of firms active in the different markets. The third paper uses the heterogeneous firm framework to study the interaction between financial constraints and the market entry behavior of firms. It also analyzes whether the impact of trade liberalization on average firm productivity and on individual welfare is affected by the presence of credit frictions. The second chapter presents an empirical work that contributes to the recent but fast growing literature that studies how different institutions and their level of development affect countries comparative advantage. The analysis presented in this paper focuses on the role of legal and financial institution in driving the specialization in contract-intensive goods and on how the degree of institutional development interacts with the propensity of firms to vertical integrate with their suppliers.