This brief from the Urban Institute describes the need for understanding and interpreting evidence for pay for success (PFS) projects and in broader public decision-making. The authors discuss the growing body of social science research showing the importance of strong evidence to determine which projects will work. This brief defines evidence, why it matters to PFS projects, how to assess the quality of existing evidence, and what to do when only limited evidence is available. The authors contend that using evidence to make public welfare decisions improves government effectiveness and drives better outcomes for society.