I shall talk about the best measured blackbody radiation at the temperature of T = (2.72548 ± 0.00057)K.
This is the cosmic microwave background which is not only a so far perfect blackbody but also very isotropic. I shall discuss about the relevance of this radiation and especially its small anisotropies and polarization for observational cosmology. I shall also present arguments why its spectrum is so stable. I shall finally address the deviation from a blackbody which we expect to see with future experiments and what we shall learn from them.