Performance of the CMS Electromagnetic Calorimeter in pp collisions

The CMS Electromagnetic Calorimeter (ECAL) is a high resolution, finely grained instrument devised to measure photons and electrons at LHC. Built of lead tungstate crystals, it will play a crucial role in the search for new physics as well as in precision measurements in the standard model. Thanks to this detector, the CMS collaboration was able to reconstruct di-photon states within minutes from the first LHC collisions. We will report on the commissioning and performance of the detector with 2009 and 2010 collision data, providing details about its calibration and synchronization, showing how well the challenging design goals were met.