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The supernova remnant Cassiopeia A contains the youngest known neutron star which is also the first one for which real time cooling has ever been observed. I n order to explain the rapid cooling of this neutron star, we first present the fundamental proper ties of neutron stars that control their thermal evolution with emphasis on the neutrino emission processes and neutron/proton superfluidity/superconductivity. Equipped with these results, we present a scenario in which the observed cooling of the neutron star in Cassiopeia A is interpreted as being due to the recent onset of neutron superfluidity in the core of the star. The manner in which the earlier occurrence of proton superconductivity determines the observed rapidity of thi s neutron star’s cooling is highlighted. This is the first direct evidence that superfluidity and super conductivity occur at supranuclear densities within neutron stars.