Physics at the LHC requires extremely high performance detectors. The CMS electromagnetic calorimeter ( ECAL) has been designed to facilitate the discovery of the Higgs boson, thought to be responsible for the spontaneous symmetry breaking observed in the electroweak sector of the Standard Model. This report outlines the particular physics requirements that govern the choice and design of the CMS ECAL and describes in some detail the properties of the Lead Tungstate crystals used in the ECAL, the readout system and the Preshower detectors. Testbeam results are presented which illustrate the progress that has been made during the past few years and which highlight the features of the CMS ECAL
