A precise measurement of luminosity is a key component of both the ATLAS and CMS physics programmes at LHC: its uncertainty is
one of the dominant systematics for both Standard Model cross section measurements and enters new physics searches. The different
strategies used by the two collaborations to assess the luminosity and the methods to absolutely calibrate it are here described,
together with the dominant systematic uncertainties. The preliminary luminosity uncertainty obtained for 2017 data is 2.4% for ATLAS
and 2.3% for CMS.