Relations among different actors can come in various forms. For example, people form friendships, academic researchers work with coauthors, corporate actors, such as firms, form strategic alliances, and states develop alliances in the context of political treaties. The overarching focus of the essays presented in this book pertains to the characteristics of individual actors (such as the industry or nationality associated with a firm), the extent to which the population of actors is heterogeneous with respect to those characteristics, and the ways in which actor characteristics and population heterogeneity influence the process of social network formation and the choices that actors make in these networks. We investigate empirical, methodological, and theoretical questions regarding social network formation in heterogeneous populations. First, we provide an empirical examination of the role of heterogeneity in the process of network formation in the context of inter-firm collaboration. Second, we study methodological issues regarding the measurement of segregation in networks. Segregation is a phenomenon that is frequently observed in social networks and an indicator of the association between population heterogeneity and network structure. Third, we address theoretical questions regarding the role of actor heterogeneity in the simultaneous dynamics of social networks and the behavior of the actors forming those networks.