Abstract : Ion-induced collisions produce athermal atomic movements at and around the surface or interface, inducing step formation and modifying growth conditions. The latter may be controlled by varying the temperature and ion-beam characteristics, guiding the system between nonequilibrium and quasiequilibrium states. Silicon is an ideal material to observe and understand such processes. For ion irradiation at or below room temperature, damage due to collision cascades leads to Si amorphization. At temperatures where defects are mobile and interact, irradiation can lead to layer-by-layer amorphization, whereas at higher temperatures irradiation can lead to the recrystallization of previously amorphized layers. This chapter focuses on the role of ion beams in the interface evolution. We first give an overview of ion beam-induced epitaxial crystallization (IBIEC) and ion-beam-induced amorphization as observed in silicon and identify unresolved issues. Similarities and differences with more familiar surface thermal growth processes are emphasized. Theories and computer simulations developed for surface relaxation help us to quantify several important aspects of IBIEC. Recent experiments provide insight into the influence of ion-induced defect interactions on IBIEC, and are also partly interpreted via computer simulations. The case of phase transformations and precipitation at interfaces is also considered.