CONTAINED IN VOL. 12.02, 2001 Outlines possible sources of error in dosimetry performed in gamma, x-ray (bremsstrahlung) and electron irradiation facilities the uncertainties in the measurement results. Basic concepts of measurement, estimate of the measured value of a quantity "true" value, error and uncertainty are discussed and methods are given for evaluating 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.