Performance on the CMS Electromagnetic Calorimeter

The Large Hadron Collider at CERN will allow the study of pp interactions at a center of mass energy of 14 TeV. The main physics goals of the Compact Muon Solenoid (CMS) experiment are the discovery of the Higgs boson and the search for new physics. Experimental conditions and physics goals set demanding constraints on the Electromagnetic Calorimeter (ECAL), made of about 76000 Lead Tungstate scintillating crystals, to be able to achieve a high precision in photon and electron energy measurement. ECAL performance has been studied using an electron test beam, before final commissioning in the experiment. The most relevant results are presented