Performance of the PWO crystals of the CMS electromagnetic calorimeter

The CMS experiment at the LHC has decided to install a homogeneous electromagnetic calorimeter made of about 80000 PWO scintillating crystals. The LHC is a very hard environment for both the radiation levels in the detectors and the high beam crossing rate, so very demanding requirements are set on the characteristics of the detector. After a long R&D the PWO crystals are now well adapted to the experiment. We review the crystal properties and their implications on the calorimeter performance. In particular we discuss the light collection crystal uniformity and its effect on the constant term of the calorimeter resolution, measured in a high energy electron beam. (7 refs).