Three neutrinos to build up the world?

The author reviews the features and specificities of neutrinos including the latest discoveries concerning oscillations and mass. The distance to which appears the first oscillation is 1500 km for 1 GeV electron neutrinos and 500 km for 1 GeV muon neutrinos. Today experimental results show 2 mass differences: Δm2 = 8.10-5 eV2 that correspond to oscillations of solar electron neutrinos or to electron neutrinos produced in nuclear reactors and Δm2 = 2.5 10-3 eV2 that agrees with oscillations of atmospheric muon neutrinos. Taking into account the limiting constraints on the neutrino mass (typically less than 1 eV/c2), neutrinos appear to be by far the lightest fermions. (A.C.)