Our present understanding of the cooling of hot neutron stars formed in supernovae collapses is briefly reviewed. The neutrino energy loss dominant in the early stages of the cooling is discussed for an equation of state involving only nucleon degrees of freedom and in the presence of a pion condensate. Assuming that the collapse of supernovae always produces hot neutron stars, it is shown that recent results from X-ray observations of young supernovae remnants by the Einstein Observatory are suggestive of the presence of pion condensates in the interior of neutron stars.