  Motivated by the effect of matter on neutrino oscillations (the MSW effect)
we have studied the propagation of neutrinos in a dense medium. The dispersion
relation for massive neutrinos in a medium is known to have a minimum at
nonzero momentum $p\sim G_F\rho/\sqrt{2}$. We have studied in detail the origin
and consequences of this dispersion relation for both Dirac and Majorana
neutrinos both in a toy model with only neutral currents and a single neutrino
flavour and in a realistic ``Standard Model'' with two neutrino flavours. For a
range of neutrino momenta near the minimum of the dispersion relation, Dirac
neutrinos are trapped by their coherent interactions with the medium. This
effect does not lead to the trapping of Majorana neutrinos.
