The mechanism of wood burning has been subjected to mathematical analysis. Extensive finite-difference computations for symmetrical and nonsymmetrical heating of flat wood slabs have provided results which appear to shed additional light upon the mechanisms leading to stable burning conditions. The results of this study indicate that the influence of source temperature upon fuel volatilization rate and surface temperature may be generalized with the aid of similitude factors. Based upon these generalized results and published experimental data, a criterion is proposed for defining conditions conducive to stable burning. This criterion establishes that such conditions exist when the total rate of volatilization of fuel or the rate of surface temperature increase becomes more rapid than that which would exist in a slab of infinite thickness. The proposed criterion is supported by published experimental data for symmetrical heating, and it is in agreement with the experimentally observed maximum ignitable thickness for one- sided heating.