A follow-up to the 2015 OECD Social Impact Investment (SII) report, this publication brings new evidence on the role of SII in financing sustainable development. It depicts the state of play of SII approaches globally, comparing regional trends, and assesses its prospects, with a special focus on data issues and recent policy developments. It provides new guidance for policy makers and providers of development co-operation to maximise the contribution of impact investing to delivering the 2030 Agenda, the Paris Agreement as well as the Addis Ababa Action Agenda. To that end, it calls for an “impact imperative” in financing for sustainable development in terms of: ensuring financing is going where it is needed most; applying innovative new approaches to solving the Sustainable Development Goals; addressing data and measurement challenges; and evaluating the ex post social, environmental and economic results of public initaitives.