Twenty-first century skills are vital for preparing youth for careers in science, technology, engineering, and math (STEM) fields. STEM out-of-school time (OST) programs play an important role in helping youth develop these skills, particularly the teamwork skills necessary for the growing collaborative nature of STEM jobs. However, there is a lack of appropriate measures to evaluate this key programmatic outcome in STEM OST settings. This dissertation research addresses the lack of measures through the development of an instrument to assess team communication skills in middle and high school