STEM out-of-school time (OST) programs play an important role in helping youth develop the 21st century skills they need to prepare them for the workforce, particularly the teamwork skills necessary for the growing collaborative nature of work in STEM (National Research Council, 2015). However, there is a lack of appropriate tools to evaluate this key programmatic outcome in STEM OST settings. Through funding from the National Science Foundation, we carried out the Collaboration in the 21st Century (C2C) project to help address this need by developing and validating a survey, the Youth