As the nation seeks ways to increase interest in science, technology, engineering, and math (STEM) education and careers, high-quality afterschool STEM programs will fill a growing need. With support from the Charles Stewart Mott Foundation and the Noyce Foundation (now STEM Next), states across the country are developing systems of support for more quality afterschool programs focused on STEM. System building elements include partnership and leadership development; evaluation and data collection activities; quality building and professional development opportunities; communication and policy; and financing and sustainability. This evaluation is among the first at a large scale to measure the impact of afterschool programs on students' STEM-related attitudes and social-emotional/21st-century skills. The primary goals of this work were (1) to examine levels of change in youth outcomes among programs receiving resources and training support from system-building states; (2) to inform on national trends related to STEM learning, such as gender or grade differences in science interest; and (3) to link STEM program quality with student outcomes and facilitator beliefs. (author abstract)