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CMS is one of the four experiments being constructed for the LHC. The electromagnetic calorimeter (ECAL) is made of lead tungstate (PbWO4) crystals. Scintillation light from the crystals in the barrel is detected by avalanche photo diodes (APD). The number of scintillation photons emitted by the crystals and the amplification of the APD are both temperature dependent. Keeping the constant term of the energy resolution (the contribution to the fractional energy resolution which is independent of energy) below a few tenths of percent requires the temperature of the the crystals and APDs to be stabilized to within 0.1 K while for example previously built L3 detector using another type of crystals had a temperature stability around 1 K which is the standard requirement for cooling systems at CERN. In this contribution, we describe the innovative solutions which have been developed and implemented to achieve this performance.