Studies of the CMS Electromagnetic Calorimeter performance in the electron test beam

The Compact Muon Solenoid (CMS) is a general purpose detector for LHC. The electromagnetic calorimeter (ECAL), comprising about 76,000 PbWO$_4$ scintillating crystals, will allow a very accurate energy measurement of electrons and photons. The 36 supermodules (units of 1700 crystals) of the barrel have been successfully integrated inside the CMS solenoid. In 2006 nine supermodules with the final mechanics, cooling and electronics were exposed to electron test beams in order to perform precise inter-calibration and comprehensive studies of the detector response. A survey of results from these tests are here presented, with emphasis given to the energy resolution and linearity achieved.