This report provides a review of stem cells/progenitor cells and their responses to ionising radiation in relation to issues relevant to stochastic effects of radiation which form a major part of the ICRP system of radiological protection. Current information on stem cell characteristics, maintenance and renewal, evolution with age, location in stem cell “niches”, radiosensitivity to acute and protracted exposures, is presented in a series of substantial reviews as Annexes concerning haematopoietic tissue, mammary gland, thyroid, digestive tract, lung, skin and bone. This foundation of knowledge of stem cells is used in the main text of the report to provide a biological insight to issues such as the linear-no-threshold (LNT) model, cancer risk among tissues, dose-rate effects and changes in the risk of radiation carcinogenesis by age at exposure and attained age.