Neutrino physics has followed a long way, full of striking discoveries, since Wolfgang Pauli postulated the existence of the ghostly particle in his famous letter to “Liebe radioaktive Damen und Herren.” The last fascinating discovery concerns the observation that neutrinos oscillate, that is, change flavor, which illustrates that the neutrino must have mass. However, oscillation experiments themselves are not able to obtain the neutrino rest mass. They can determine only the mass differences squared. The absolute mass must be determined by different measurements.