SiH3 radicals created by electron impact dissociation of SiH4 in reactive gas discharges are widely believed to be the dominant precursor for plasma deposition of amorphous and nanocrystalline silicon thin films. In this article, we present a systematic computational analysis of the interactions of SiH3 radicals with a variety of crystalline and amorphous silicon surfaces through atomistic simulations. The hydrogen coverage of the surface and, hence, the availability of surface dangling bonds has the strongest influence on the radical-surface reaction mechanisms and the corresponding reaction probabilities. The SiH3 radical reacts with unit probability on the pristine Si(001)-(2 × 1) surface which has one dangling bond per Si atom; upon reaction, the Si atom of the radical forms strong Si-Si bonds with either one or two surface Si atoms. On the H-terminated Si(001)-(2 × 1) surface, the radical is much less reactive; the SiH3 radical was reflected back into the gas phase in all but two of the 16 simulations of radical impingement designed to sample the high-symmetry adsorption sites on the surface. When SiH3 reacts on the H-terminated surface, it either inserts into the Si-Si dimer bond or returns to the gas phase as SiH4 after abstracting H from the surface. The insertion into the Si-Si bond occurs through a dissociative adsorption reaction mechanism that produces two surface SiH2 species after transfer of one of the H atoms from SiH3 to one of the dimer Si atoms. The energetics and dynamics of the surface reactions are analyzed in detail. During simulations of a-Si:H film growth, adsorption onto a dangling bond, dissociative insertion, and H abstraction reactions also were observed to occur with similar energetics as the corresponding reactions on crystalline surfaces. The radical is much more mobile on surfaces of a-Si:H films than crystalline surfaces, especially when the hydrogen concentration in the amorphous film and, thus, on the surface is high.