The Three-Neutrino Mixing and Oscillations

The basics of neutrino oscillations is presented. Existing evidences of neutrino oscillations, obtained in the atmospheric and solar neutrino experiments, are considered. The new CHOOZ bound on the element $\mathrm{\mid }{U}_{e3}{\mathrm{\mid }}^2$, obtained from the three-neutrino analysis of the data, is discussed. Decoupling of neutrino oscillations in the solar neutrino range of $\mathrm{\Delta }{m}^2$ is considered.