Chris Fox presented a paper based on a structured review of emprical evaluations of 32 Social Impact Bonds (SIBs) and 35 Payment by Results (PbR) programmes implemented in the UK since 2010 at a conference on Progressing the debate on Social Impact Bonds: Theoretical and empirical developments in the analysis of Social Impact Bonds. The paper explored the evidence base underpinning SIBs and PbR and discussed how the evaluation of such programmes might be undertaken in future. The paper is an output from an ongoing project reviewing the effectiveness of outcome-based payment models. Slides from the presentation are available to download on the project page.