Afterschool programs have increasingly gained attention as settings that can help enrich students' science learning (Halpern, 2004). Even though science is widely included in afterschool activities, sites often lack adequate materials and staff know-how to implement quality science. Moreover, not much guidance is available on how afterschool sites can offer quality science within the practical constraints of their work (Chi, Freeman, & Lee, 2008; Noam et al., 2010). To address this need, this article examines afterschool science in light of the National Research Council's comprehensive synthesis report on promoting science learning in informal environments (NRC, 2009). We present the results of our analysis of qualitative case studies of nine state-funded afterschool sites in California, discussing the strengths of these programs against the background of three key site-based constraints--time available for science, staff's science backgrounds, and instructional materials--as well as the importance of partnerships with outside organizations to support sites in overcoming these obstacles. (author abstract)