Index funds own an increasingly large proportion of American public companies. The stewardship decisions of index fund managers—how they monitor, vote, and engage with their portfolio companies—can be expected to have a profound impact on the governance and performance of public companies and the economy. Understanding index fund stewardship, and how policymaking can improve it, is thus critical for corporate law scholarship. In this Article we contribute to such understanding by providing a comprehensive theoretical, empirical, and policy analysis of index fund stewardship.We begin by putting forward an agency-costs theory of index fund incentives. Stewardship decisions by index funds depend not just on the interests of index fund investor...