In situ decommissioning (ISD) is the permanent entombment of a contaminated facility. At present, ISD is not recognized or addressed in the Department of Energy (DOE) and Office of Environmental Management (EM) lexicon; however, ISD is not a revolutionary concept. Since the 1970s, the U.S. Nuclear Regulatory Commission (NRC) has recognized the option of entombing a facility as a decommissioning option. Permanent entombment of a radioactively contaminated facility as a decommissioning option has been completed for one facility at the Idaho National Laboratory and is currently planned at a limited number of selected DOE facilities. The regulatory framework is currently in place to provide assurance that the risk posed by an ISD facility is within regulatory acceptance criteria.