Abstract; The operation and performance of the Compact Muon Solenoid; (CMS) electromagnetic calorimeter (ECAL) are presented, based on; data collected in pp collisions at; √; s; =13 TeV at the CERN LHC, in the years from 2015 to 2018; (LHC Run 2), corresponding to an integrated luminosity of; 151 fb-1. The CMS ECAL is a scintillating lead-tungstate; crystal calorimeter, with a silicon strip preshower detector in the; forward region that provides precise measurements of the energy and; the time-of-arrival of electrons and photons. The successful; operation of the ECAL is crucial for a broad range of physics goals,; ranging from observing the Higgs boson and measuring its properties,; to other standard model measurements and searches for new; phenomena. Precise calibration, alignment, and monitoring of the; ECAL response are important ingredients to achieve these goals. To; face the challenges posed by the higher luminosity, which; characterized the operation of the LHC in Run 2, the procedures; established during the 2011–2012 run of the LHC have been revisited; and new methods have been developed for the energy measurement and; for the ECAL calibration. The energy resolution of the calorimeter,; for electrons from Z boson decays reaching the; ECAL without significant loss of energy by bremsstrahlung, was; better than 1.8%, 3.0%, and 4.5% in the -η- intervals; [0.0,0.8], [0.8,1.5], [1.5, 2.5], respectively. This resulting; performance is similar to that achieved during Run 1 in 2011–2012,; in spite of the more severe running conditions.
