A physics-based forest fire model, based on a multiclass description of two-phase flow, is developed to study fire behavior and the response of structures to fire-induced thermal stress. The model is three-dimensional and considers the coupled physicochernical processes that take place in both phases: the thermal degradation of organic matter and glowing combustion of the char, as well as turbulence, flaming combustion, soot formation, and radiation for the gas phase. Model results are compared with data from two specially designed experiments. The first refers to a back-wind prescribed burning over a 900 m(2) area of steep-slope terrain. The model predicts not only the mean rate of fire spread, but also the convex shape of the head-fire front resulting from three-dimensional effects. In the second experiment, attention is focused on the thermal impact of a fire-exposed structural element placed in a wind tunnel. The predicted fire-front trajectory is shown to be in good agreement with measurements as well as the temperature level and the location of the exposed area of the structure. (c) 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved.