Electromagnetic calorimetry and e/

The electromagnetic calorimeter of the CMS experiment at LHC will consist of about 76000 Lead Tungstate crystals. Its main purpose is the very precise energy measurement of electrons and photons produced in proton–proton collisions at 14 TeV centre-of-mass energy. A review is given of its main building blocks and its construction status. Then the results of the testbeam campaign in 2002 are discussed. Finally, the overall calibration and reconstruction strategy is outlined. PACS: CMS ECAL electromagnetic calorimetry crystal