  The optical module of the KM3NeT neutrino telescope is an innovative,
multi-faceted large area photodetection module. It contains 31 three-inch
photomultiplier tubes in a single 0.44 m diameter pressure-resistant glass
sphere. The module is a sensory device also comprising calibration instruments
and electronics for power, readout and data acquisition. It is capped with a
breakout-box with electronics for connection to an electro-optical cable for
power and long-distance communication to the onshore control station. The
design of the module was qualified for the first time in the deep sea in 2013.
Since then, the technology has been further improved to meet requirements of
scalability, cost-effectiveness and high reliability. The module features a
sub-nanosecond timing accuracy and a dynamic range allowing the measurement of
a single photon up to a cascade of thousands of photons, suited for the
measurement of the Cherenkov radiation induced in water by secondary particles
from interactions of neutrinos with energies in the range of GeV to PeV. A
distributed production model has been implemented for the delivery of more than
6000 modules in the coming few years with an average production rate of more
than 100 modules per month. In this paper a review is presented of the design
of the multi-PMT KM3NeT optical module with a proven effective background
suppression and signal recognition and sensitivity to the incoming direction of
photons.
