Neutron stars present a unique opportunity to observe high density, low temperature nuclear matter. Cooling processes in neutron stars probe neutrino physics, the nuclear equation of state, superfluidity, phase transitions, and other properties of nuclear matter. X-ray observations can constrain the neutrino emissivity of accreting systems. In particular, observations of relatively cold systems with large neutrino emissivity indicates the presence of fast cooling mechanisms. In this talk, I will review basic features of neutrino cooling in neutron stars and show that the x-ray transient SAX J1808 requires the presence of fast cooling processes. Should a transition to deconfined quark matter occur at neutron star densities, this requirement puts limits on the critical temperature of different pairing channels in color superconducting quark matter.