The U.S. Department of Energy's Hanford Site was established in southeastern Washington to produce plutonium during World War II. The Pantex Plant in the Texas Panhandle was originally used for loading conventional ammunition shells and bombs, but was rehabilitated and enhanced in the 1950s to assemble nuclear weapons. Environmental monitoring has been ongoing at both locations for several decades. Monitoring objectives are to detect and assess potential impacts of facility operations on air, surface and ground waters, foodstuffs, fish, wildlife, soils, and vegetation. Measured concentrations of airborne radionuclides around the perimeters of both sites are currently below applicable guidelines. Concentrations of radionuclides and nonradiological water quality in the Columbia River at Hanford, and radiological and nonradiological water quality in the Ogallala Aquifer beneath the Pantex Plant are in compliance with applicable standards. Foodstuffs irrigated with river water downstream from the Hanford Site show levels of radionuclides that are similar to those found in foodstuffs from control areas. The low levels of 137Cs and 90Sr in some onsite Hanford wildlife samples and concentrations of radionuclides in soils and vegetation from onsite and offsite at both locations are typical of those attributable to naturally occurring radioactivity and to worldwide fallout. The calculated dose potentially received by a maximally exposed individual (i.e., based on hypothetical, worst-case assumptions for all routes of exposure) at both sites in 1994 was ≤0.05 mrem. Ironically, by virtue of its size (1,450 km2 [560 mi2]), restricted public access, and conservative use of undeveloped land, the Hanford Site has provided a sanctuary for plant and animal populations that have been eliminated from, or greatly reduced on, surrounding agricultural and range lands. Ongoing studies will determine if this is also true at Pantex Plant.