Payment by results allows the government to pay a provider of services on the basis of the outcomes their service achieves rather than the inputs or outputs the provider delivers. Social impact bonds (SIBs) are a form of payment by results which allow the financing of social outcomes via private investment. It is suggested that payment by results and SIBs will drive greater efficiency, innovation and impact in tackling social problems through focusing reward on outcomes and providing minimal prescription as to how these outcomes should be achieved. It is suggested that this may be achieved while also reducing risk for government. Here the authors set out the challenges likely to arise in developing payment by results models and SIBs in the criminal justice system of England and Wales. These include the uncertainty arising from defining outcomes, estimating the potential impact of interventions, measuring and attributing change, valuing benefits, demonstrating a fiscal return and getting interventions to scale. The authors conclude that, to a government trying to deliver 'more for less', payment by results may offer an attractive solution in some parts of the public sector. However, the case for this approach in the criminal justice sector, where the evidence base is contested and potential savings difficult to quantify and realize, is not yet proven. (Published Abstract)