This monograph provides a comprehensive overview of the role of institutional investors in corporate governance. There are three main components: 1) tracing the emergence of institutional investors as the modal concentrated owners of public firms in modern economies; 2) providing a detailed characterization of key aspects of the legal and regulatory setting within which institutional investors operate with respect to the governance of their portfolio firms; and 3) synthesizing the evolving academic literature in finance to the emergence of institutional investors in corporate governance and linking theoretical predictions to empirical findings. Following an introduction, section 2 provides a series of new stylized facts on the evolution of institutional ownership and its heterogeneity in the US and outside of the US. Section 3 describes the legal and regulatory environment within which institutional investors operate, with a focus on the obligations, ability, and incentives of such investors to engage in the corporate governance of firms. Section 4 reviews the theoretical literature on institutional investors and corporate governance while Section 5 discusses the empirical literature. Section 6 provides conclusions.