In the standard model the three lepton flavours are separately conserved. The observation of neutrino oscillations and thus neutrino masses clearly established the violation of two linear combinations of the lepton flavour numbers. This implies the existence of charged lepton flavour violating processes, where charged leptons change flavour in absence of neutrinos. These processes exist, but are extremely small if induced by the three active light neutrinos due to unitarity of the leptonic mixing matrix and the smallness of neutrino masses.