Evidence is at the core of the pay for success movement, which pushes government to rigorously evaluate programs and pay only for those that achieve positive outcomes. Evidence, however, is only as good as the evaluation that produces it. As such, evaluators are an integral part of any PFS project, from beginning to end. In these projects, the evaluator implements activities that assess program outcomes in order to determine success payments from government to investors. Evaluators should be engaged early in PFS project development. Their early involvement can inform overall project design in meaningful ways, by, for example, increasing the ability to rigorously assess outcomes and the confidence in payment schemes. The purpose of this paper is to introduce practical considerations for evaluators when integrating established, rigorous evaluation methods within the structure of a PFS project.