6846 20240222141517.0 SCART-2024-0096 Groenewegen, M.A.T. Primary Period–Luminosity-Relation Calibrators in the Milky Way: Cepheids and RR Lyrae: Physical basis, Calibration, and Applications 2024 In this invited review I discuss the calibration and applications of the period– luminosity relation of classical Cepheid and RR Lyrae stars. After a brief introduction, starting with results from Hipparcos and discussing some post Hipparcos era developments, I focus on recent results using Gaia Data Release 3 data. I present an overview of the most recent period–luminosity relations, a discussion and some new results on Cepheids in open clusters. I also discuss the effect of reddening and that the use of Wesenheit indices is actually an oversimplification to dealing with the problem of reddening. NO Proceedings IAU Symposium No. 376 2024 128-149 martin.groenewegen@ksb-orb.be 681738 http://publi2-as.oma.be/record/6846/files/primary-period-luminosity-relation-calibrators-in-the-milky-way-cepheids-and-rr-lyrae-physical-basis-calibration-and-applications.pdf 18325 http://publi2-as.oma.be/record/6846/files/primary-period-luminosity-relation-calibrators-in-the-milky-way-cepheids-and-rr-lyrae-physical-basis-calibration-and-applications.gif?subformat=icon icon 22489 http://publi2-as.oma.be/record/6846/files/primary-period-luminosity-relation-calibrators-in-the-milky-way-cepheids-and-rr-lyrae-physical-basis-calibration-and-applications.jpg?subformat=icon-180 icon-180 published in REFERD