The CMS electromagnetic calorimeter design goal is to be precise and accurate enough to detect the Higgs → 2 photons signal over a continuous background. A resolution better than 1% for showers of 50-100 GeV is required. To achieve this goal, all the factors that can eventually spoil the calorimeter resolution have to be taken into account. This includes: careful measurements and control of crystal properties before installation, precise calibration of each crystal with an electron beam before use in CMS and accurate off-line reconstruction of the electromagnetic showers. The importance of these items and the progress achieved will be discussed.