Updating the National Fire-Danger Rating System (NFDRS) was completed in 1977, and operational use of it was begun the next year. The System provides a guide to wildfire control and suppression by its indexes that measure the relative potential of initiating fires. Such fires do not behave erratically-they spread without spotting through continuous ground fuels. Estimates of fire potential have a basis in the mathematical models used for fire behavior. The fire manager must select the fuel model that best represents the fuels in the preotection area. Among the 20 fuel models available, not more than two or three are appropriate for any one area. This documentation of the 20 fuel models and their equations supplements previous reports on the System. The equations are presented in the coded format of FORTRAN and BASIC computer languages.