Cesium-137 (137Cs) is the most important long-term contributor to the environmental radiation dose received by humans and other organisms as a result of nuclear reactor operations and weapons testing. Over the past few decades, 137Cs has been the most abundant residual radionuclide at many facilities in the nuclear weapons complex of the U.S. Department of Energy (DOE), at nuclear fuel reprocessing facilities, at nuclear reactor sites, at many radioactive waste disposal sites, in soils worldwide as a result of global fallout from historic nuclear weapons testing, and in the former Soviet Union and other locales in Europe as a result of the Chernobyl accident. In addition, there is concern about the use of 137Cs by terrorists to create a so-called “dirty bomb.”