The processes by which atoms in solids are displaced from their normal positions by high-energy neutrons, cyclotron particles, and electrons are discussed. A radiation damage model is presented which involves two basic features: (1) the production of interstitial atoms and vacant lattice sites, and (2) the production of displacement spikes. These two concepts are sufficient to account for most of the observed property changes resulting from atomic displacement by high-energy particles. Some of the experimental results which provide the most direct support for these concepts are presented.