Social Impact Bonds (SIBs) are spreading around the world and have been gathering increasingly the attention of governments and public authorities, investors, social services providers, researchers, and evaluators among others over the last years. At the same time, they have triggered debates -often controversial- around issues such as the delivery of social services and the quest for efficiency in doing so, the risk transfer from the public to private sector and what this entails for social services providers, the capacity to monitor and evaluate better outcomes, and the increasingly prevalent need to invest in preventative interventions with high returns in the long run. SIBs tend to have strong proponents or strong opponents. Common ground among all, however, was the need for more evidence in order to assess their potential in an informed way.