This dissertation consists of three essays. The first essay "Network Games of Imperfect Competition: An Empirical Framework" studies how product differentiation affects the ability of firms to price above costs in oligopolistic markets where products are differentiated over multiple attributes. It shows that, under suitable assumptions, firms' pricing power is fully summarized by a measure of network centrality function of product characteristics. The second essay "Plan Menus, Retirement Portfolios and Investors' Welfare" studies the US market of employer-sponsored retirment plans. It develops a structural model of plan menu design and fee competition between funds to rationalize the incentives behind the provision of high-cost investment options and to assess the effects of alternative plan design policies on investors’ welfare. The third essay "Oligopolistic Competition, Fund Proliferation, and Asset Prices" (joint work with Federico Mainardi) develops and estimates a dynamic oligopoly model to understand why different mutual fund families introduce new funds at different rates and at different points in time and how these competitive dynamics impact asset prices and investors’ welfare.