This International Standard specifies the minimum requirements for the evaluation of data from the monitoring of workers occupationally exposed to the risk of internal contamination by radioactive substances. It presents procedures and assumptions for the standardised interpretation of monitoring data, in order to achieve acceptable levels of reliability. Those procedures allow the quantification of exposures for the documentation of compliance with regulations and radiation protection programmes. Limits are set for the applicability of the procedures in respect of the dose levels above which more sophisticated methods will have to be applied. This International Standard addresses the following: a) procedures for dose assessment based on reference levels for routine and special monitoring programmes; b) assumptions for the selection of dose-critical parameter values; c) criteria for determining the significance of monitoring results; d) interpretation of workplace monitoring results; e) uncertainties arising from sampling, measurement techniques and working conditions; f) the special topics of 1) interpretation of multiple data arising from different measurement methods at different times, 2) handling data below the decision threshold, 3) rogue data, and 4) calculation of doses to the embryo/foetus and infant; g) reporting/documentation; h) quality assurance. It is not applicable to the following: - dosimetry for litigation cases; - modelling for the improvement of internal dosimetry; - the potential influence of decorporation measures (e.g. administration of chelating agents); - the investigation of the causes or implications of an exposure; - dosimetry for contaminated wounds.