The increase in institutional ownership of recent decades has been accompanied by an enhanced role played by institutions in monitoring companies’ corporate governance behaviour. Activist hedge funds and private equity firms have achieved a degree of success in actively shaping the business plans of target firms. They may be characterized as pursuing a common goal – in the words used in the OECD Steering Group on Corporate Governance, both seek ‘to increase the market value of their pooled capital through active engagement with individual public companies. This engagement may include demands for changes in management, the composition of the board, dividend policies, company strategy, company capital structure and acquisition/disposal plans which are normally regarded as governance issues.’ This article is the introductory chapter of Institutional Investor Activism: Hedge Funds and Private Equity, Economics and Regulation (Oxford University Press 2015). The book collects descriptive expositions and empirical analyses essential for an understanding of both varieties of interventionist shareholder. The twenty-one chapters detail these investors’ strategic approaches, the financial returns they produce, the regulatory context in which they operate, and the policy questions raised by their activities.