Performance of lead tungstate crystals for the CMS electromagnetic calorimeter

A short description of the general characteristics of electromagnetic calorimeters (ECAL) and the motivation for their use in High Energy Physics is made. The quality requirements to the Lead Tungstate PbWO //4 (PWO) crystals to be used for the CMS-ECAL barrel and endcaps construction are summarised. Some of the test beam results concerning the PWO performance are presented. Other recent results of the studies performed on the PWO crystals used for ECAL modules construction are discussed