When the Universe was from about 1 second to 1 minute old, many interesting processes took place. On the one hand, weak interactions became inefficient and relic neutrinos decoupled from the cosmic plasma. And, at the far end, Big Bang Nucleosynthesis kicked off and the first light nuclei were formed. In between, electrons and positrons annihilated into photons, effectively reheating them with respect to the already decoupled neutrinos. The outcome of all these processes can affect the evolution of the Universe from that time until what we know today. In this talk I will focus on the decoupling process of relic neutrinos, starting with the standard scenario in which we properly accounted for flavour oscillations. Then I will explain how the effective number of relativistic species, also known as effective number of neutrinos (Neff) is modified assuming some non-standard scenarios. In this way, comparing the standard value of Neff with present and future observations we can get some insight about physics beyond the Standard Model, from the presence of non-standard interactions in the neutrino sector to the possibility of having an extra (sterile) neutrino species.