  From a phenomenological point of view, we study active-active and
active-sterile flavour-changing (and flavour-conserving) oscillations of
Dirac-Majorana neutrinos both in vacuum and in matter. The general expressions
for the transition probabilities in vacuum are reported. We then investigate
some interesting consequences following from particular simple forms of the
neutrino mass matrices, and for the envisaged scenarios we discuss in detail
neutrino propagation in matter. Special emphasis is given to the problem of
occurrence of resonant enhancement of active-active and active-sterile neutrino
oscillations in a medium. The peculiar novel features related to the
Dirac-Majorana nature of neutrinos are particularly pointed out.
