This book analyses 10 C.F.R. 37, a new rule promulgated by the Nuclear Regulatory Commission (NRC) to protect radioactive material. "By-product material" includes specified types of radioactive material other than uranium or plutonium. The rule regulates by-product material of types and in quantities that could be used to make a "dirty bomb". Congress attaches great importance to protecting the United States against terrorist threats and this rule will affect the many industrial, research, and medical activities nation-wide that use radioactive materials, thereby affecting many constituents and raising cost-benefit issues. There is wide concern also about regulation and radiation.