Neutrino oscillations imply that the Standard Model is incomplete and must be extended to include neutrino mass as well as mixing among the three neutrino flavours. The mechanism by which neutrino mass is generated is not known. An intriguing possibility is that the tiny neutrino mass is the result of physics at extremely high energy scales. Such a “see-saw” mechanism might also help to explain why neutrino mixing is so much stronger than the mixing among quarks. Mixing among three massive neutrinos admits the possibility that symmetry between matter and antimatter (CP-symmetry) is violated via the neutrino mixing matrix. Nonzero neutrino mass implies that lepton number must be used to distinguish a neutrino from an antineutrino. If lepton number is not conserved then a neutrino is indistinguishable from an antineutrino, i.e. the neutrino is a Majorana particle – a completely new state of matter. The determination of the properties of the neutrino, therefore, is fundamental to the development of particle physics.