We address issues of science curriculum for nonmainstream students--students of color, students learning English as a new language, and students from low-income families--who are often concentrated in urban schools. First, we describe a theoretical framework for equitable learning opportunities with nonmainstream students. Building on this framework, we then discuss challenges in designing and implementing science curriculum materials for these students. Although some of these challenges affect nonmainstream students more broadly, other challenges are more directly related to specific student groups. Next, we provide examples of curriculum development and research programs to illustrate key components in the theoretical framework and to highlight how these programs address challenges in curriculum design and implementation. Finally, we offer an agenda to guide future research and development efforts. We discuss how alternative, sometimes competing, theoretical views of curriculum development in the literature can be brought together in the context of high-stakes testing and accountability policy.