Direct numerical Simulation with detailed chemistry and transport is used to study the stabilization and dynamics of lean (phi = 0.5) premixed hydrogen/air atmospheric pressure flames in mesoscale planar channels. Channel heights of h = 2, 4, and 7 mm, and inflow velocities in the range 0.3 <= U-IN <= 1100 cm/s are investigated. Six different burning modes are identified: mild combustion, ignition/extinction, closed steady symmetric flames, open steady symmetric flames, oscillating and, finally, asymmetric flames. Chaotic behavior of cellular flame structures is observed for certain values Of U-IN. Stability maps delineating the regions of the different flame types are finally constructed. (c) 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.