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The CMS electromagnetic calorimeter aims at providing high precision calorimetry at the large madron collider (LHC). It consists of about 75.000 lead tungstate (PbWO4) crystals that have to operate reliably for at least 10 years, in a high radiation environment. In order to profit fully from the very good intrinsic energy resolution of the crystals, severe requirements on their temperature stability must be satisfied. Another challenge is given by the high data rates to be sustained by the readout electronics. With more than a half of the barrel modules produced, the calorimeter is well into its production phase. A large effort was devoted to optimize the integration between mechanics, cooling and readout electronics. The performance of the first modules has been tested in an electron beam, to validate the monitoring system and the calibration strategy. Very satisfactory results were achieved, in complete agreement with the goals of ECAL. An overview of the calorimeter design and of its construction status will be given, as well as the results from the testbeam measurements and the predicted performance at the LHC.