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 ^ { 3 } P - F _ { 2 } energy gap of the order of 0.1 MeV .