Financial economics and corporate governance have long focused on the agency problems between corporate managers and shareholders that result from the dispersion of ownership in large publicly traded corporations. In this paper, we focus on how the rise of institutional investors over the past several decades has transformed the corporate landscape and, in turn, the governance problems of the modern corporation. The rise of institutional investors has led to increased concentration of equity ownership, with most public corporations now having a substantial proportion of their shares held by a small number of institutional investors. At the same time, these institutions are controlled by investment managers, which have their own agency problems vis-à-vis their own beneficial investors. We develop an analytical framework for understanding the agency problems of institutional investors, and apply it to examine the agency problems and behavior of several key types of investment managers, including those that manage mutual funds—both index funds and actively managed funds—and activist hedge funds. We show that index funds have especially poor incentives to engage in stewardship activities that could improve governance and increase value. Activist hedge funds have substantially better incentives than managers of index funds or active mutual funds. While their activities may partially compensate, we show that they do not provide a complete solution for the agency problems of other institutional investors.