Exposure to radioactivity is not a new phenomenon: Jewelry workers painting luminous dials on watches in the 1920s were exposed to radium, a naturally occurring radioactive element, until its dangers were identified; in recent decades, homeowners have worried about radon, a naturally occurring radioactive gas that can adversely affect both their health and the resale value of their homes. Today, we must worry as well about the enormous quantities of anthropogenic radioactive materials that have accumulated as waste in this country and worldwide since the 1940s, largely because of the nuclear weapons programs and nuclear power plants. Some of the anthropogenic material is still valuable—for example, as fuel for nuclear reactors. Vast amounts of it, however, consist of waste. Radioactive waste, by definition, is unneeded material that contains unstable elements that decay by emitting alpha, beta or gamma emissions. This article describes the sources of this waste, the types of waste that are of concern and the amounts estimated to be present in the US.