[en] Two modeling studies were conducted to simulate the effect of fallout shelters on the outcome of a massive countervalue nuclear exchange between the Soviet Union and the United States. One was to determine the number of nuclear weapons required to mount an effective fallout attack against a country with dispersed population; the other was to determine the number of expected US fatalities resulting from a countervalue attack against US urban population centers. The results of these studies indicate that the number of weapons required to mount such an attack depends on the adequacy of the shelter system and that the evacuation of urban populations can substantially reduce expected fatality levels