Science, technology, engineering, and mathematics (STEM) programs in out-of-school time (OST) are designed to supplement school work, ignite student interest, and extend STEM learning. From interactive museum exhibits to summer-long science camps, opportunities for informal student engagement in STEM learning abound. The differences these programs make, and how they can be improved concern educators and funders alike. OST program developers and providers can benefit from understanding why evaluation is critical to the success of STEM OST programs, what data collection methods are appropriate, and how to effectively communicate and report findings. The authors of this article share lessons from their experience in each of these areas and provide examples of how effective practices play out.