  Neutron Compton scattering measurements have the potential to provide direct
information about atomic momentum distributions and adiabatic energy surfaces
in condensed matter. First applied to measuring the condensate fraction in
superfluid helium, the technique has recently been extended to study a variety
of classical and quantum liquids and solids. This article reviews the
theoretical background for the interpretation of neutron Compton scattering,
with emphasis on studies of solids.
