  In this talk we review the results on neutrino propagation under external
magnetic fields. We concentrate on the effects of strong magnetic fields
$M_{W}^{2}\gg B\gg m_{e}^{2}$ in neutral media. It is shown that the neutrino
energy density get a magnetic contribution in the strong-field, one-loop
approximation, which is linear in the Fermi coupling constant as in the charged
medium. It is analyzed how this correction produces a significant oscillation
resonance between electron-neutrinos and the other two active flavors, as well
as with sterile neutrinos. The found resonant level-crossing condition is
highly anisotropic. Possible cosmological applications are discussed. Effects
due to primordial hypermagnetic fields on neutrinos propagating in the
symmetric phase of the electroweak model are also presented. At sufficiently
strong hypermagnetic fields, $B\geq T^{2}$, the neutrino energy is found to be
similar to that of a massless charged particle with one-degree of freedom.
