An atomistic model for the transformation of amorphous (&agr;) to crystalline silicon films while in contact with a crystalline substrate is presented. The atomic structure of the {100}, {110}, and {111} surfaces is examined and related to the observed interface migration rates. The assumption that for an atom to attach successfully to the crystal it must complete at least two undistorted bonds, leads to the prediction that the {100} amorphous/crystalline interface should advance fastest and the {111} slowest. The origin of crystal defects is discussed in terms of the atomistic recrystallization mechanism. Microtwins are found to be a logical consequence of crystallization on the {111} surfaces but are not expected to form on any other interface. Once microtwins are formed they can increase the recrystallization rate of a {111} surface. This phenomenon is both described in the model and experimentally observed.