This paper aims to provide an introduction to and overview of the social investment market for policy makers in OECD and non-OECD countries. Social investment is the provision of finance to organisations with the explicit expectation of a social, as well as financial, return. Social investment has become increasingly relevant in today’s economic environment as social challenges have mounted while public funds in many countries are under pressure. New investment approaches are needed for addressing social and economic challenges, including new models of public and private partnership which can fund, deliver and scale innovative solutions from the ground up.