This International Standard specifies methods for the determination of absorbed-dose distributions in water or tissue that are required prior to initiating procedures for the application of beta radiation in ophthalmic tumour and intravascular brachytherapy [1], [2], [3]. Recommendations are given for beta radiation source calibration, dosimetry measurements, dose calculation, dosimetric quality assurance, as well as for beta radiation brachytherapy treatment planning. Guidance is also given for estimating the uncertainty of the absorbed dose to water. This International Standard is applicable to “sealed” radioactive sources, such as plane and concave surface sources, source trains of single seeds, line sources, and shell and volume sources, for which only the beta radiation emitted is of therapeutic relevance. The standardization of procedures in clinical dosimetry described in this International Standard serves as a basis for the reliable application of beta radiation brachytherapy. The specific dosimetric methods described in this International Standard apply to sources for the curative treatment of ophthalmic disease, for intravascular brachytherapy treatment, for overcoming the problem of restenosis and for other clinical applications using beta radiation. This International Standard is geared towards organizations wishing to establish reference methods in dosimetry aiming at clinical demands for an appropriately small uncertainty of the delivered dose. This International Standard does not exclude the possibility that there can be other methods leading to the same or smaller measurement uncertainties.