  The properties of the extremely strong magnetic fields of neutron stars
affect in a unique way their evolution and the associated phenomenology. Due to
the lack of constraints from direct observations, our understanding of the
magnetic field configuration in neutron star interiors depends on the progress
in theoretical modelling. Here we discuss the effort in building models of
magnetized neutron stars focussing on some of the recent results. In
particular, we comment on the instability of purely poloidal and purely
toroidal magnetic field configurations and on the evidence in favour of the
so-called twisted-torus solutions. We conclude with an outlook on the present
status of the field and future directions.
