  Different models based on the extended $SU(3)_{C}\times SU(3)_{L}\times
U(1)_{X}$ (331) gauge group have been proposed over the past four decades. Yet,
despite being an active research topic, the status of lepton number in 331
models has not been fully addressed in the literature, and furthermore many of
the original proposals can not explain the observed neutrino masses. In this
paper we review the basic features of various 331 models, focusing on potential
sources of lepton number violation. We then describe different modifications
which can be made to the original models in order to accommodate neutrino (and
charged lepton) masses.
