A fire growth model, FARSITE (Fire ARea SImulator) is under development for simulating the spread and behavior of prescribed natural fires. The models uses a technique for wave propagation to expand surface fire fronts in 2 dimensions. Points defining the outer edge of a surface fire contain data on the time, direction of spread, and rate of spread. These parameters provide a logical basis for implementing models of crown fire, fire exceleration, and spotting. Preliminary validation of the FARSITE model against surface fire spread patterns from 5 prescribed natural fires suggest good agreement after some adjustments. Observations suggest consistant overprediction of spread rates for each fuel type.