Social Impact Bonds (SIBs) represent a new way to finance social service and health promotion programs whereby different types of investors provide an upfront investment of capital. If a given program meets predetermined criteria for a successful outcome, the government pays back investors with interest. Introduced in the United Kingdom in 2010, SIBs have since been implemented in the United States and across Europe, with some uptake in other jurisdictions. We identify and explore selected areas of concern related to SIBs, drawing from literature examining market-based reforms to health and social services and the evolution of the SIB funding mechanism. These areas of concern include increased costs to governments, restricted program scope, fragmented policymaking, undermining of public-sector service provision, mischaracterization of the root causes of social problems, and entrenchment of systemically produced vulnerabilities. We argue that it is essential to consider the long-term, aggregate, and contextualized effects of SIBs in order to evaluate their potential to contribute to public health. We conclude that such evaluations must explore the assumptions underlying the "common sense" arguments often used in support of SIBs.