  ALICE is an LHC experiment devoted to the study of strongly interacting
matter in proton-proton, proton--nucleus and nucleus-nucleus collisions at
ultra-relativistic energies. The ALICE VZERO system, made of two scintillator
arrays at asymmetric positions, one on each side of the interaction point,
plays a central role in ALICE. In addition to its core function as a trigger,
the VZERO system is used to monitor LHC beam conditions, to reject beam-induced
backgrounds and to measure basic physics quantities such as luminosity,
particle multiplicity, centrality and event plane direction in nucleus-nucleus
collisions. After describing the VZERO system, this publication presents its
performance over more than four years of operation at the LHC.
