This chapter provides an overview of the most prominent characteristics of Social Impact Bonds (SIBs) by focusing, in particular, on the implication that the development of the market may have for both the public and private sectors. It describes the role of financial institutions and the importance of the development of impact metrics and standards of evaluations. The chapter also describes the major characteristics of SIBs and the main challenges that should be addressed for specific market development. It shows that the SIB research field needs further development and proposes a research agenda consisting of four main themes: interdisciplinary and to borrow from the domains of public policy, accountability, and finance. The Rikers Island SIB was based on the general goal of permanently reducing the number of jail beds operated and, thus, saving money per bed. Banks can play an essential role both in the financial structuring of a SIB and in providing this instrument to investors.