The CMS electromagnetic calorimeter comprises over 70 000 lead-tungstate crystals and a pre-shower silicon detector in the endcap sections. The goal of achieving 0.5% for the constant noise term has presented important technological challenges, like a temperature stability of 0.01C, or following the crystal transparency to better than 0.5%. Also in order to keep up with the 100 kHz level 1 trigger rate in CMS, the detector's readout includes a special system that decides which regions need to be fully read out in order to preserve the energy resolution. This seminar will cover aspects of the conception, R&D, construction and commissioning of the CMS ECAL during the last 20 years and the first physics results directly issuing from its use.
