This publication is intended to support physicists working in the field of radiotherapy dosimetry, both in standards laboratories involved in the calibration of dosimeters and in clinical centres and hospitals where external beam radiotherapy is utilized. The report fulfils the need for an internationally unified approach to the calibration of ionization chambers in terms of air kerma and absorbed dose to water in the low and medium energy X ray and cobalt-60 beams used for external beam radiotherapy. This publication also includes the most recent international guidance on quality management systems applicable to standards laboratories.