The electromagnetic calorimeter of CMS: Status and performance with cosmic rays data and first LHC data

The CMS detector at the LHC is ready to take physics data. The high resolution electromagnetic calorimeter, which consists of 75 848 lead tungstate crystals, will play a crucial role in the physics searches undertaken by the CMS. The design and the status of the calorimeter will be presented, and its performance in tests with beams, cosmic rays, and data from the first LHC beams will be reviewed.