  Collective excitations in magnetic materials can be investigated by means of
inelastic neutron scattering. We show that this experimental method gives
access to the complete spectrum of magnetic fluctuations through the energy-
and momentum-dependence of the dynamical susceptibility. We focus on the
dynamical properties of magnets with localized spin densities and of metals.
  From such studies, microscopic parameters like exchange integrals, spin-wave
stiffness and relaxation times can be determined. This is of great help to test
current theories in magnetism.
