"A social impact bond (SIB) is a new approach for scaling social programs. Currently being piloted in the United Kingdom and generating interest globally, a SIB is a multistakeholder partnership in which philanthropic funders and impact investors—not governments—take on the financial risk of expanding preventive programs that help poor and vulnerable people. Nonprofits deliver the program to more people who need it; the government pays only if the program succeeds. Because the concept of a SIB is so new (the first and only SIB is the UK pilot mentioned above), information about how—and how well—this approach could work is very limited. In this report, the most thoroughly researched study of SIBs to date, we explain how SIBs are structured, assess their potential in two specific program areas (homelessness and criminal justice), describe the various stakeholder groups involved, and present the results of a pro forma analysis of a hypothetical SIB."