This International Standard specifies the minimum requirements for the design of professional programmes to monitor workers exposed to the risk of internal contamination by radioactive substances and establishes principles for the development of compatible goals and requirements for monitoring programmes. This International Standard addresses the a) purposes of monitoring and of 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) special cases; i) quality assurance; and j) documentation, reporting, record-keeping. This International Standard does not address the monitoring of exposure to radon and its radioactive decay products; detailed descriptions of measuring methods and techniques; detailed procedures for in vivo measurements and in vitro analyses; interpretation of monitoring results in terms of doses; biokinetic data and mathematical models for converting measured activities into absorbed dose, equivalent dose and effective dose; or the investigation of the causes or implications of an exposure or intake.