  I review some theoretical aspects of neutrino oscillations in the case when
more than two neutrino flavours are involved. These include: approximate
analytic solutions for 3-flavour (3f) oscillations in matter; matter effects in
nu_mu - nu_tau oscillations; 3f effects in oscillations of solar, atmospheric,
reactor and supernova neutrinos and in accelerator long-baseline experiments;
CP and T violation in neutrino oscillations in vacuum and in matter; the
problem of U_{e3}; 4f oscillations.
