Hydrogenated amorphous silicon films for photovoltaics and thin film transistors are deposited from silane containing discharges. The radicals generated in the plasma such as SiH3 and H impinge on the surface and lead to silicon film growth through a complex network of elementary surface processes that include adsorption, abstraction, insertion and diffusion of various radicals. Mechanism and kinetics of these reactions determine the film composition and quality. Developing deposition strategies for improving the film quality requires a fundamental understanding of the radical-surface interaction mechanisms. We have been using in situ multiple total internal reflection Fourier transform infrared spectroscopy and in situ spectroscopic ellipsometry in conjunction with atomistic simulations to determine the elementary surface reaction and diffusion mechanisms. Synergistic use of experiments and atomistic simulations elucidate elementary processes occurring on the surface. Herein, we review our current understanding of the reaction mechanisms that lead to a-Si:H film growth with special emphasis on the reactions of the SiH3 radical.