For protection purposes, the biological effects of radiation are separated into stochastic effects (cancer, hereditary effects) presumed to be unicellular in origin, and tissue reactions due to injury in populations of cells. The latter are deterministic effects, renamed ‘tissue reactions’ in the 2007 Recommendations of the International Commission on Radiological Protection because of the increasing evidence of the ability to modify responses after irradiation. Tissue reactions become manifest either early or late after doses above a threshold dose, which is the basis for recommended dose limits for avoiding such effects. Latency time before manifestation is related to cell turnover rates, and tissue proliferative and structural organisation. Threshold doses have been defined for practical purposes at 1% incidence of an effect. In general, threshold doses are lower for longer follow-up times because of the slow progression of injury before manifestation. Radiosensitive individuals in the population may contribute to low threshold doses, and in the future, threshold doses may be increased by the use of various biological response modifiers post irradiation for reducing injury. Threshold doses would be expected to be higher for fractionated or protracted doses, unless doses below the threshold dose only cause single-hit-type events that are not modified by repair/recovery phenomena, or if different mechanisms of injury are involved at low and high doses.