  The physics motivation, detector design, triggers, calibration, alignment,
simulation, and overall performance of the very forward CASTOR calorimeter of
the CMS experiment are reviewed. The CASTOR Cherenkov sampling calorimeter is
located very close to the LHC beam line, at a radial distance of about 1 cm
from the beam pipe, and at 14.4 m from the CMS interaction point, covering the
pseudorapidity range of $-$6.6 $\lt$ $\eta$ $\lt$ $-$5.2. It was designed to
withstand high ambient radiation and strong magnetic fields. The performance of
the detector in measurements of forward energy density, jets, and processes
characterized by rapidity gaps, is reviewed using data collected in proton and
nuclear collisions at the LHC.
