The Planning Tool was created to help science, technology, engineering, and mathematics (STEM)-providing out-of-school time (OST) programs engage with families to support youth learning and success in STEM, but the key ideas and strategies are also relevant for schools. This tool such programs in effectively and equitably engaging families—especially those from underserved communities—in their children’s STEM learning. It builds on research and field experience, emphasizing that family attitudes and involvement significantly impact children’s STEM engagement and success.