Yucca Mountain Science and Engineering Report describes the results of scientific and engineering studies of the Yucca Mountain site, the waste forms to be disposed, the repository and waste package designs, and the results of the most recent assessments of the long-term performance of the potential repository. The scientific investigations include site characterization studies of the geologic, hydrologic, and geochemical environment, and evaluation of how conditions might evolve over time. These analyses considered a range of processes that would operate in and around the potential repository. Since projections of performance for 10,000 years are inherently uncertain, the uncertainties associated with analyses and models of long-term performance are also described, along with the likely impact of these uncertainties on performance assessment.