There are two main biological effects of ionising radiation: deterministic and stochastic. Deterministic effects are those in which the number of cells lost in an organ or tissue is so great that there is a loss of tissue function. The harm will not occur below a threshold and above this the severity of the effect will increase with dose. Skin erythema and ulceration are examples of deterministic effects. Stochastic effects occur if an irradiated cell is modified rather than killed and then goes on to reproduce. The result may be the manifestation of a cancer after a prolonged and variable delay called the latent period. Stochastic effects do not appear to have a threshold and the probability of the effect occurring is related to the radiation dose. The International Commission for Radiation Protection (ICRP), an international professional body, produce recommendations which are the basis for the conceptual framework and key principles of radiation protection world wide. The current international recommendations1 (ICRP 60) lay out the principles of justification, optimisation, and limitation of all radiation exposures. While there are no specified dose limits for patients, the main principle is that all deterministic effects should be avoided while optimising radiation exposure to gain maximum diagnostic accuracy with minimum dose.