Neutrinos play unique roles in many epochs of the Universe’s evolution. Important information can be gleaned from neutrino evolution during the big bang nucleosynthesis (BBN) era, for example, the best limit on the Universe’s lepton number results from considering BBN constraints together with large angle neutrino mixing. At later times neutrinos have a significant impact on the cosmic microwave background and large scale structure power spectra. Cosmological data now place the tightest constraints on neutrino mass, though there is scope to evade these limits if the physics of the neutrino sector is non-standard. We outline the wealth of information that can be revealed by studying the sea of relic neutrinos.