This article describes efforts to develop educators, in both formal and informal settings, who possess theories of teaching and learning that not only encompass but, perhaps, depend on bridging institutional and contextual boundaries. It describes the experiences of The Exploratorium, a museum in San Francisco, King's College London, and the University of California Santa Cruz, in a National Science Foundation-funded project. The article begins with a discussion of the growing interest that science education decision makers and policymakers show in bridging formal and informal settings and resources to make learning in K-12 science more engaging, authentic, and conceptually rich. It then describes how CILS designed programs and partnerships to support the development of educators to have not only expanded views of learning, but also experience and facility with designing and leading programs that draw on features and affordances of both formal and informal science settings. The authors close with a discussion of the types of evidence and understanding that are required to expand and sustain educational partnerships that span formal and informal boundaries.