This International Standard specifies the minimum requirements for the design of professional programmes to monitor workers exposed to the risk of internal contamination via inhalation by the use of radionuclides as unsealed sources in nuclear medicine imaging and therapy departments. It establishes principles for the development of compatible goals and requirements for monitoring programmes and, when adequate, dose assessment. It presents procedures and assumptions for the risk analysis, for the monitoring programmes, and for the standardized interpretation of monitoring data. This International Standard addresses the following items: a) purposes of monitoring and monitoring programmes; b) description of the different categories of monitoring programmes; c) quantitative criteria for conducting monitoring programmes; d) suitable methods for monitoring and criteria for their selection; e) information that has to be collected for the design of a monitoring programme; f) general requirements for monitoring programmes (e.g. detection limits, tolerated uncertainties); g) frequencies of measurements; h) procedures for dose assessment based on reference levels for routine and special monitoring programmes; i) assumptions for the selection of dose-critical parameter values; j) criteria for determining the significance of individual monitoring results; k) interpretation of workplace monitoring results; l) uncertainties arising from dose assessments and interpretation of bioassays data; m) reporting/documentation; n) quality assurance. This International Standard does not address the following: — monitoring and internal dosimetry for the workers exposed to laboratory use of radionuclides such as radioimmunoassay techniques; — monitoring and internal dosimetry for the workers involved in the operation, maintenance, and servicing of PET cyclotrons; — detailed descriptions of measuring methods and techniques; — dosimetry for litigation cases; — modelling for the improvement of internal dosimetry; — the potential influence of medical treatment of the internal contamination; — the investigation of the causes or implications of an exposure; — dosimetry for ingestion exposures and for contaminated wounds.