This document comprises a reference manual for computer programs (FIREMODS) pertaining to wildfire behavior and its effects, maintained by the Fire Fundamental research work unit, Northern Forest Fire Laboratory, Missoula, Montana. The subroutines embody mathematical models that permit the prediction of the linear rate of advance of a fire, perimeter and area growth rate, intensity, flame length, area fire intensity, fuel consumption, and duff burnout, under specifiable conditions of wind, slope, and fuel moisture. The routines are written in FORTRAN IV, extended, and stored in UPDATE (a CDC file-maintenance utility program) format. The algorithms are described in terms of input, output, organization, and the purpose and precision of calculations. Where the theory is not documented elsewhere, considerable detail is given on the mathematical bases for the calculations performed. Examples of input and output are included.