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. What difference do these programs make, and how can we improve them? These questions preoccupy 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