  Neutrino event generators are an essential tool needed for the extraction of
neutrino mixing parameters, the mass hierarchy and a CP violating phase from
long-baseline experiments. In this article I first describe the theoretical
basis and the approximations needed to get to present-days generators. I also
discuss the strengths and limitations of theoretical models used to describe
semi-inclusive neutrino-nucleus reactions. I then confront present day's
generators with this theoretical basis by detailed discussions of the various
reaction processes. Finally, as examples I then show for various experiments
results of the generator GiBUU for lepton semi-inclusive cross sections as well
as particle spectra. I also discuss features of these cross sections in terms
of the various reaction components, with predictions for DUNE. Finally, I argue
for the need for a new neutrino generator that respects our present-day
knowledge of both nuclear theory and nuclear reactions and is as much
state-of-the-art as the experimental equipment. I outline some necessary
requirements for such a new generator.
