Congress has long sought through legislation and oversight to protect the United States against terrorist threats, especially from chemical, biological, radiological and nuclear (CBRN) weapons. Radiological dispersal devices (RDDs) are one type of CBRN weapon. Explosive-driven "dirty bombs" are an often-discussed type of RDD, though radioactive material can also be dispersed in other ways. Radioactive material is the necessary ingredient for an RDD. This material is composed of atoms that decay, emitting radiation. Some types and amounts of radiation are harmful to human health. This book provides background for understanding the RDD threat and responses, and presents issues for Congress.