Many physics analyses using the Compact Muon Solenoid (CMS) detector at the LHC require
accurate, high resolution electron and photon energy measurements. Particularly important are
the decays of the Higgs boson resulting in electromagnetic particles in the final state, as well as the
searches for very high mass resonances decaying into energetic photons or electrons. Following
the excellent performance achieved in Run I at center-of-mass energies of 7 and 8 TeV, the CMS
electromagnetic calorimeter (ECAL) is operating at the LHC with proton-proton collisions at 13
TeV center-of-mass energy. The instantaneous luminosity delivered by the LHC during Run II
has achieved unprecedented values, using 25 ns bunch spacing. High pileup levels necessitate a
retuning of the ECAL readout and trigger thresholds and reconstruction algorithms, to maintain
the best possible performance in these more challenging conditions. The energy response of
the detector must be precisely calibrated and monitored to achieve and maintain the excellent
performance obtained in Run I in terms of energy scale and resolution. A dedicated calibration
of each detector channel is performed with physics events exploiting electrons from W and Z
boson decays, photons from $\pi^0$/$\eta$ decays, and from the azimuthally symmetric energy distribution
of minimum bias events. This contribution describes the calibration strategies and the performance of
the CMS ECAL throughout Run II and its role in precision physics measurements with CMS
involving electrons and photons.