The ways in which young people learn about science, technology, engineering, and mathematics (STEM) has fundamentally changed in the past decade. More so than ever, young people now have opportunities to learn STEM in a wide variety of settings, including clubs, summer programs, museums, parks, and online activities. They spend more time in supervised programs outside of school, and they have greater access to on-demand learning resources and opportunities. At the same time, STEM learning outside of school has become a focal piece of the education opportunities provided by many national nonprofit organizations, statewide education networks, federal programs, and corporate and family foundations. And there is growing evidence that opportunities to learn STEM outside of school directly affect what is possible inside classrooms, just as what happens in classrooms affects out-of-school learning. The Committee on Successful Out-of- School STEM Learning was charged with outlining the criteria that policy makers, program developers, and other stakeholders can use to identify effective out-of-school STEM settings and programs. It was also charged with identifying those criteria for which data are readily available and those for which further work is needed to develop appropriate data sources. To address its charge, the committee organized a National Summit on Successful Out-of-School STEM Learning, reviewed relevant research, and commissioned papers to synthesize existing research.