Pay for Success (PFS), also known as Social Impact Bonds, is a new contracting mechanism to fund prevention programs, where investors provide capital to implement a social service -- for example, to reduce recidivism by former prisoners. If the service provider achieves agreed upon outcomes, the government pays the investor, usually with a rate of return, based on savings from decreased use of more costly remedial services, such as incarceration. Federal, state, and local agencies play an important role in improving social outcomes for society's most vulnerable populations. A small number of state, local, and foreign governments are employing PFS to fund efforts designed to better serve these vulnerable populations. This book examines how selected PFS projects have been structured and what potential benefits these projects can provide; how selected PFS contracts have been structured to address potential project risks; and the potential roles for the federal government's involvement in PFS projects. This book also provides an overview of the first State-led Pay for Success and Social Impact "Bond" (SIB) project in the nation; examines some of the arguments for and against The Payment by Results (PbR) approach to delivery of public services; and looks at current and planned projects in rehabilitation, welfare to work, the NHS, children's social services and with rough sleepers and with vulnerable young people.