This thesis contains three papers which examine the role of networks and social structure in different modes of socio-economic interactions. The first chapter focuses on purely competitive strategic bilateral interactions - contests. I analyse situations in which agents, embedded in a network, simultaneously play interrelated bilateral contest games with their neighbours. The network structure uniquely determines the behaviour of agents in the equilibrium. I also study the formation of such networks, finding that the complete k-partite network is the unique stable network topology. This implies that agents will endogenously sort themselves in partitions of friends, competing with members of other partitions. The model provides a micro-foundation for the structural balance concept in social psychology, and the main results go in line with theoretical and empirical findings from other disciplines, including international relations, sociology and biology. The second chapter is joint work with my supervisor Fernando Vega-Redondo. We study a competitive equilibrium model on a production network of firms, identifying the measure of centrality in the network that determines the profit of a firm, and network structures that maximize social welfare. The significant part of this chapter focuses on how the network mediates the effects of revenue distortions on profits of firms and social welfare. The results are that the effects of distortions propagate both upstream and downstream through the network. The centrality of the affected firm determines the magnitude of the downstream effect, and the upstream effect is determined by the intercentralities of suppliers of the affected firm. Increasing the density of the network by adding links has a non-monotonic effect on welfare. Adopting a more complex production technology can increase but also decrease the profit of a firm, depending on the network structure; while finding a new buyer will always increase the profit of a firm. In the third paper I analyse the interaction between formal legal enforcement of cooperation and the role of reputation in a heterogeneous population. By choosing to cooperate, even when the quality of the formal institution is not high, an agent signals that he has high work ethics, thereby earning reputation as a better match for future interactions. When there is reputation benefit, the welfare-maximizing quality of the enforcement institution is generally not the one that maximizes cooperation. Depending on the distribution of types in society, the effect of the increase in quality of enforcement on cooperation can be crowded in or crowded out by reputation concerns. When the institutional quality is determined endogenously, the equilibrium quality of the institution will generically be higher than the optimal quality.