A one-dimensional unsteady model is formulated for biomass gasification in a stratified concurrent (downdraft) reactor. Heat and mass transfer across the bed are coupled with moisture evaporation, biomass pyrolysis, char combustion and gasification, gas-phase combustion and thermal cracking of tars. Numerical simulation has allowed to predict the influence of model parameters, kinetic constants and operational variables on process dynamics, structure of the reaction front and quality of the producer gas. In particular, two different stabilization modes of the reaction front have been determined. For high values of the air-to-fuel ratio and of the primary pyrolysis rate, the process is top-stabilized, resulting in a high conversion efficiency and good gas quality. As the air flow is decreased below a critical limit value, the reaction front becomes grate-stabilized. The two different configurations are largely determined by the gas-phase combustion of volatile pyrolysis products. Finally, the predictions of the gas composition and the axial temperature profiles are in agreement with experimental data.