Evaluations are a key feature of pay for success (PFS) projects, and rigorous evaluation designs are important for building the evidence base of effective programs by determining whether a project’s outcomes can be attributed to the program. Randomized controlled trials (RCTs) are considered the most rigorous evaluation design and give us the best approximation for what would have happened without the program. However, PFS stakeholders often don’t know about RCTs or consider them too expensive, difficult, or controversial. This brief outlines RCTs, their advantages, and solutions to overcoming perceived and real challenges to their use in the context of PFS.