NRC's Offices of Nuclear Regulatory Research (RES) and Federal and State Materials and Environmental Management Programs (FSME) organized this Workshop on Engineered Barrier Performance Related to Low-Level Radioactive Waste, Decommissioning, and Uranium Mill Tailings Facilities. The workshop was held August 3-5, 2010 at the U.S. Nuclear Regulatory Commission (NRC) Headquarters Auditorium, 11545 Rockville Pike, Rockville, Maryland. The Workshop was coordinated with the States (i.e., Texas, South Carolina, Utah, Colorado, Washington, and New York), Tribal Nations (Navajo, Umatilla and Nez Perce), and Federal agencies (e.g., U.S. Department of Energy DOE], U.S. Environmental Protection Agency EPA], U.S. Department of Agriculture's Agricultural Research Service USDA/ARS], U.S. Geological Survey USGS], and DOE National Laboratories). The workshop technical topics focused on engineered surface covers and bottom liners designed to isolate waste by impeding surface-water infiltration into the waste systems and mitigating the migration of contaminants from the waste disposal site. Topics included engineered barrier performance, modeling, monitoring, and regulatory experiences at low-level radioactive waste, decommissioning, and uranium mill tailings sites. The workshop objectives included: (1) facilitation of communication among Federal and State staff and contractors and selected experts on current engineered barrier issues and technical and regulatory experiences; (2) discussion of lessons learned and approaches for monitoring and modeling; (3) preparation of recommendations to address maintenance of engineered barrier performance over time; and (4) identification of topics for future research and the potential need to update technical guidance. Recommendations and insights given during session presentations, panel debates, and the discussions that followed were documented by the session reporters and are included in this report.