An accurate measurement of the delivered luminosity is a key component of the ATLAS physics programme. For cross-section measurements, the uncertainty in the delivered luminosity is often one of the major systematic uncertainties. Searches for, and eventual discoveries of, physical phenomena beyond the Standard Model also rely on accurate information about the delivered luminosity to evaluate background levels and determine sensitivity to the signatures of new phenomena. This seminar presents the measurement of the luminosity delivered to the ATLAS detector at the LHC in pp collisions.