This publication covers all the aspects of the relative biological effectiveness (RBE) of ion beams, including laboratory measurements of RBE and the important variables that influence it, dose related quantities and units, and approaches to the clinical use of the concept of RBE based on experimental findings, theoretical models, and previous clinical experience with fast neutrons and ions. This publication is the result of a joint initiative of the IAEA and ICRU (International Commission on Radiation Units and Measurements). It is the only current extensive review of ion beam RBE, and it is expected to be a reference volume for existing and future centres employing ion beams for therapeutic use.