Abstract. This paper is intended to give information about the on-going work of the MetrExtRT research project concerned with the dosimetric metrology of modern external beam radiotherapy. It started on the 1 st ofJune 2012 for duration of 36 months and covers the study medium-and high-energy X-rays, high-energy electrons, electronic brachytherapy and scanned proton and carbon ion beams. The project concerns all the steps of traceability from the primary standards through to verification of the dose in and around the tumour, namely: developing new primary standards of absorbed dose to water; studying new detectors and improving the knowledge of the characteristics of existing detectors, useable for quality control and in vivo dosimetry, and publishing guide lines of good practice on their use. MetrExtRT gathers 10 partners that are all National Metrology Institutes (NMI) or Designated Institutes (DI) as represented by the list of authors, two Researcher Excellence Grants (REG) for the development of passive gel dosimetry (Université d'Auverge -France) and diamond detectors (Tor Vergata University -Italy) and up to now 12 collaborators from industry, oncology centres and universities.