  We study the Berry phase in neutrino oscillations for both Dirac and Majorana
neutrinos. In order to have a Berry phase, the neutrino oscillations must occur
in a varying medium, the neutrino-background interactions must depend on at
least two independent densities, and also there must be CP violation if the
neutrino interactions with matter are mediated only by the standard model W and
Z boson exchanges which implies that there must be at least three generations
of neutrinos. The CP violating Majorana phases do not play a role in generating
a Berry phase. We show that a natural way to satisfy the conditions for the
generation of a Berry phase is to have sterile neutrinos with active-sterile
neutrino mixing, in which case at least two active and one sterile neutrinos
are required. If there are additional new CP violating flavor changing
interactions, it is also possible to have a non-zero Berry phase with just two
generations.
