Hydrogen (H) diffusion and evolution effects in hydrogenated amorphous and microcrystalline silicon (Si) films are discussed. Numerous diffusion and evolution related effects have been discovered and various models for H motion, trapping and evolution effects have been proposed. Microstructure and H diffusion and evolution effects are found to be strongly interdependent. Various aspects of H diffusion in compact material are consistent with a trapping/detrapping motion of atomic H, restricted by H solubility effects and describable in terms of an equilibrium band model. (C) 2002 Elsevier Science B.V. All rights reserved.