The social sector's hottest "impact investing" product--the social impact bond (SIB)--has generated a range of reactions, from excitement to angst. An SIB uses private funds to pay for a social, educational, or health program, and the government repays investors (plus a return) only if the program achieves prespecified results. The intervention financed by the first social impact bond in the United States--the Adolescent Behavioral Learning Experience (ABLE) program at Rikers Island jail in New York City--was discontinued after three years when results from an independent evaluation demonstrated that it was not meeting its goal of reducing recidivism among 16- to 18-year-olds. As president of MDRC--the organization that was the "intermediary" at the center of the Rikers deal--Gordon Berlin thinks it is important that the field not learn the wrong lessons from the experience. Instead, emerging lessons from the Rikers deal and others reveal both SIBs' value to government entities and also the reality that this value will only be realized if the tensions inherent in structuring the terms of an SIB deal can be addressed squarely. These lessons include: (1) the balance of risk and reward; (2) the focus on government savings; (3) the tyranny of SIB metrics; and (4) the role of evidence. This paper draws on lessons from the implementation of the ABLE program and the handful of other SIBs with actual operating experience to provide valuable insights into the inner workings of SIB deals, using ABLE as a vehicle for explaining both the challenges and the potential of SIBs. It provides an overview of the SIB promise, and briefly describes the ABLE project. The paper frames critical decisions and identifies central tensions in the design of a deal, including the unique requirements of due diligence in an SIB project, and walks through the steps involved in structuring a deal, showing how the tensions inherent in the SIB mechanism must be confronted. Recommendations for moving beyond a series of "bespoke" deals to effective, ongoing programs operating on a large scale, whether through sustained government funding or through the creation of functioning capital markets that afford a continuing role for private investors, especially philanthropies, are included.