Pay for Success (PFS), a form of social impact financing, is receiving international attention as a way to pay for scaling up high-return interventions, ranging from prisoner rehabilitation to infant health. It is attractive because risk of failure is shifted from taxpayers to the private sector; if programs don’t work, government doesn’t pay. Government pays for success by rebating a large portion of the savings from programs that work to private investors in those programs. If there are no savings, that is if interventions do not reduce government costs, there is nothing to rebate to investors. In this article, I review contracting and time-to-completion considerations with particular attention to feasibility studies, the critical first stage of establishing a social impact finance program.