In recognition of the fact that science centers and other informal educational institutions can play a role in the reform of science, technology, engineering, and mathematics (STEM) education, several major research and professional programs are currently underway. This article discusses one such effort, the Center for Informal Learning and Schools (CILS), a collaboration of the Exploratorium, the University of California, Santa Cruz, and King's College, London and the need for a theoretical framework based on socio-cultural theory to link discussion of varied efforts characterizing science learning in informal settings. The article discusses two key problematics related to developments in the science education field of the past decade: (1) integrating studies that are undertaken from multiple disciplinary perspectives, namely, science education, developmental psychology, and cultural studies, and (2) characterizing critical properties of informal learning in museums. It reviews work that has been conducted in nonschool settings and, using examples from research conducted by the Center for Informal Learning and Schools, it reviews questions currently under investigation. © 2004 Wiley Periodicals, Inc. Sci Ed88(Suppl. 1):S71–S82, 2004