The CMS Electromagnetic Calorimeter (ECAL) is a high resolution, finely grained calorimeter devised to measure photons and electrons at LHC. Built of lead tungstate crystals, it plays a crucial role in the search for new physics as well as in precision measurements in the Standard Model. The status and general performance of the CMS ECAL in proton-proton collisions 2010 run at √s = 7TeV are described. The precision of the inter-channel calibration and absolute energy scale has been verified and improved exploiting in situ data. The quality of the offline
data reconstruction, from low level quantities to high level objects, has been investigated and improved using known physics processes. Collision data and data/MC
comparisons have been used to measure and tune the detector performance
