Top quark pair production cross section at CMS

Precision measurements of the top-pair production cross-section in proton-proton collisions at the LHC at a centre-of-mass energy of 7 and 8 TeV are presented. The data are collected with the CMS experiment during the years 2011 and 2012. The measurements are performed in several decay channels: the dilepton, the lepton plus jets, and the fully hadronic channels, including the $\tau$-dilepton and $\tau$ plus jets modes. B-jet identification is used to increase the purity of the selection. The backgrounds are determined using data-driven techniques. The results are combined with each other and compared with theory predictions.