Neutrino oscillations

Neutrino oscillations occur if neutrinos have masses and if they mix among each other. This phenomenon would be in close analogy to the experimentally observed mixing in the quark sector. Neutrino oscillations are therefore very interesting in the context of elementary particle physics. Further neutrino oscillation in matter is probably the reason for the observed deficit of the solar neutrino flux. As an interference effect, the search for neutrino oscillations is the only way to investigate even very small differences of neutrino masses. In the present article, after an introduction of the theoretical background concerning massive neutrinos, the authors describe important experiments which have been performed in this field. Experiments looking for neutrino oscillations have been carried out using accelerators, nuclear power plants, cosmic rays and the Sun as neutrino sources. Also the observed neutrino burst of supernova SN 1987A has implications for the oscillation parameter. The authors show the interesting link of neutrino oscillations to related phenomena, like neutrino decay and the possible existence of a 17 keV neutrino, coupled weakly to the electron.