We discuss the phenomenology of neutrino decoupling in the early universe in standard and non-standard scenarios. We review how the effective number of neutrino species in the early universe (N$_{\rm eff}$) is calculated in the three-neutrino case, which gives Neff=3.044, and show how this result can change when one considers non-standard properties such as non-standard interactions between neutrinos and electrons and a non-unitary three-neutrino mixing matrix.