Protecting nuclear material from violent use against society has been an issue at the very heart of the nuclear energy debate from its beginning. A review was made to determine the effectiveness of the Department of Defense's (DOD) nuclear safeguards system and to compare that system to the systems used by the Department of Energy (DOE) and the Nuclear Regulatory Commission (NRC) who are also responsible for the security of nuclear weapons or nuclear weapons-grade materials against theft, diversion, or sabotage. In addition, the on-site security at a number of sites within the responsibility of each of these agencies was investigated by interviewing site officials and employees, touring the sites, and by testing various elements of the security system. DOD is responsible for the security of nuclear weapons and for the security of nuclear weapons-grade materials found in research reactor fuel and in fuel for naval propulsion reactors. In carrying out this responsibility, DOD has established minimum standard security requirements that are to be met by its nuclear facilities. At five weapon sites that were reviewed, it was discovered that while all of the sites met most of the minimum requirements, each one needed important improvements before all of the requirements would be met. With regard to DOD's research reactor and naval fuel storage facilities, the security appeared to be adequate. Some minor improvements could be made, but their importance to the overall security of the facility was not significant. Like DOD, NRC and DOE are also responsible for protecting the public against the hazards of theft, diversion, or sabotage of nuclear materials. While all three agencies are responsible for large amounts of nuclear materials, DOD and DOE also have custody of nuclear weapons themselves. Each agency designs its security system to what it believes to be the existing threat, and each has different estimates of what that threat is. These different threat levels have resulted in a situation where similar nuclear materials are receiving different levels of protection.