  In this review, I present a brief summary of the impact of nucleon pairing at
supra-nuclear densities on the cooling of neutron stars. I also describe how
the recent observation of the cooling of the neutron star in the supernova
remnant Cassiopeia A may provide us with the first direct evidence for the
occurrence of such pairing. It also implies a size of the neutron 3P-F2 energy
gap of the order of 0.1 MeV.
