The thesis is organized as follows. Chapter 2 contains a survey of the three most in‡fluential models on fi…rm heterogeneity and of the most important empirical work on firrm heterogeneity. The chapter starts with a brief review of the homogeneous productivity imperfect competition literature. Chapter 2 …finishes with a comparison of the three most in‡fluential models of fi…rm heterogeneity and the oligopoly model put forward in the thesis. Chapter 3 addresses exporting uncertainty under heterogeneous popularity. Chapter 4 contains the chapter on …firm heterogeneity under oligopoly. Chapter 5 constitutes the models on …firm heterogeneity and endogenous quality. Chapter 6 points out the within-sector specialization model. Chapter 7 addresses the effect of importer characteristics on unit values and the role of markups and quality to explain this effect. Chapter 8 concludes.

Access to relevant external knowledge is crucial for a firms' competitiveness in innovation-driven industries. This thesis focuses on how different forms of proximity affect a firm's ability to access such knowledge. We consider the influence of being co-located in space, of being embedded in a network, and of being active in similar knowledge domains. By integrating these three proximity perspectives, we contribute to various disciplines such as economic geography, organizational sociology and innovation studies. Further, we investigate the make, buy or ally strategies that pharmaceutical firms employ to maximize the probability of innovation (finding new drugs). Our findings suggest that firms employ multiple governance structures simultaneously, even when targeting similar innovations. These insights contribute to our understanding of the boundaries of the firm.

In most decisions we have to choose between options that involve some uncertainty about their outcomes and their effect on our well-being. Casual observation and carefully controlled studies suggest that, in making these decisions, we often deviate from the benchmark of expected income maximization. This should not come as a surprise. Our well-being is affected by many factors, and the outside observer does not know the importance of various dimensions of the outcome to the decision maker. Even if goals are well defined, it is far from obvious that we succeed in choosing what is best for us. The psychological literature has shown deviations from optimal behavior in simple decision tasks, and we may expect similar deviations to occur in more complex real life problems. In real life situations, however, experience and market interaction will help to restrain suboptimal behavior. This thesis examines deviations from expected income maximization in situations involving uncertainty. We focus on deviations generated by social factors.