1 This guide defines possible sources of error in dosimetry performed in gamma, x-ray (bremsstrahlung) and electron irradiation facilities and offers procedures for esti-mating the resulting magnitude of the uncertainties in the measurement results. Basic concepts of measurement, esti-mate of the measured value of a quantity, "true" value, error and uncertainty are defined and discussed. Components of uncertainty are discussed and methods are given for evalu-ating and estimating their values. How these contribute to the standard uncertainty in the reported values of absorbed dose are considered and methods are given for calculating the combined standard uncertainty and an estimate of overall (expanded) uncertainty. The methodology for evalu-ating components of uncertainty follows ISO procedures (see 2.3). The traditional concepts of precision and bias are not used. Examples are given in five appendixes. 2 This guide assumes a working knowledge of statistics. Several statistical texts are included in the references (1, 2,3, 4). 3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.