  Strong (B >> 10^9 G) and superstrong (B > 10^{14} G) magnetic fields
profoundly affect many thermodynamic and kinetic characteristics of dense
plasmas in neutron star envelopes. In particular, they produce strongly
anisotropic thermal conductivity in the neutron star crust and modify the
equation of state and radiative opacities in the atmosphere, which are major
ingredients of the cooling theory and spectral atmosphere models. As a result,
both the radiation spectrum and the thermal luminosity of a neutron star can be
affected by the magnetic field. We briefly review these effects and demonstrate
the influence of magnetic field strength on the thermal structure of an
isolated neutron star, putting emphasis on the differences brought about by the
superstrong fields and high temperatures of magnetars. For the latter objects,
it is important to take proper account of a combined effect of the magnetic
field on thermal conduction and neutrino emission at densities \rho > 10^{10} g
cm^{-3}. We show that the neutrino emission puts a B-dependent upper limit on
the effective surface temperature of a cooling neutron star.
