The Full Option Science System (FOSS), developed at the Lawrence Hall of Science, has been adopted by hundreds of school districts across the United States. FOSS districts range in size from a single, K–6 elementary school to huge, urban-centered districts serving thousands of students. Even so, the differences among FOSS users are greatly overshadowed by the characteristics common to the successful district implementations. Feedback from a group of "case study" sites indicates that four dimensions of the implementation process are critical—staff development, materials management, administrative support, and communication.