The general theory of neutrino masses and mixing is introduced. The theory of neutrino oscillations in vacuum and in matter is reviewed, with discussion of the most important general phenomenological aspects. Topics: - Dirac Neutrino Masses Higgs Mechanism in the Standard Model Three-Generation Neutrino Masses and Mixing - Majorana Neutrino Masses Two-Component Theory of a Massless Neutrino Majorana Mass Mixing of Three Majorana Neutrinos - Dirac-Majorana Mass One Generation See-Saw Mechanism Three-Generation Mixing - Neutrino Oscillations in Vacuum General Theory Two-Neutrino Mixing and Oscillations - Neutrino Oscillations in Matter Effective Potentials in Matter MSW Effect (Resonant Transitions in Matter) Phenomenology of Solar Neutrinos