The strongest magnetic fields observed in the Universe can be found in
neutron stars which manifest themselves across the whole
electromagnetic spectrum as pulsars and a collective class known as
magnetars. While the former are powered by their rotation, the latter
are thought to extract the emitted energy from their magentic field
itself. In both cases, the magnetic field is the domninate factor in
the spin-evolution of these sources and is likely to be responsible for
many or even most of the observed properties. This talk will review the
basic properties and knowledge of magnetic fields in neutron stars and
how they can be inferred and observed.