This review article provides an overview, for medical physicists, of recent work that addresses the role of nuclear reactions in proton, fast neutron, and conventional photon radiation therapy. Nuclear reaction cross sections have been evaluated, in ENDF format, for use in Monte Carlo radiation transport simulations of radiotherapy, in order to optimize the absorbed dose to a tumor. The evaluations utilize both measured cross section data as well as nuclear model calculations of direct, preequilibrium, and Hauser-Feshbach compound processes. In the case of photonuclear reactions, giant dipole resonance and quasideuteron photoabsorption mechanisms are considered.