The Department of Energy's Office of Environmental Management is developing a technology roadmap to guide planning and possible future congressional appropriations for its technology development programs. It asked the National Research Council of the National Academies to provide technical and strategic advice to support the development and implementation of this roadmap, specifically by undertaking a study that identifies principal science and technology gaps and their priorities for the cleanup program based on previous National Academies reports, updated and extended to reflect current site conditions and EM priorities and input form key external groups, such as the Nuclear Regulatory Commission, Defense Nuclear Facilities Safety Board, Environmental Protection Agency, and state regulatory agencies. In response, this book provides a high-level synthesis of principal science and technology gaps identified in previous NRC reports in part 1. Part 2 summarizes a workshop meant to bring together the key external groups to discuss current site conditions and science and technology needs.