With the start of Run-2 of the LHC in 2015 physical processes predicted by the Standard Model are now being tested at 13 TeV centre-of-mass energy. In particular, the cross section of W and Z bosons in proton-proton collisions can be measured with high accuracy. The inclusive and differential cross sections are compared with predictions from higher-order calculation in QCD and the electroweak sector and parton distribution functions can be experimentally evaluated. A leading systematic uncertainty in the measurements is the normalization of the dataset, the integrated luminosity. A dedicated accelerator setup, so called van-der-Meer scans, enable precise estimates of the proton beam properties. I will present the important aspects of luminosity measurements at the LHC and the current status of the W and Z boson cross section measurements with CMS.