  In the last two decades experiments have established the existence of
neutrino oscillations and most of the related parameters have by now been
measured with reasonable accuracy. These results have accomplished a major
progress for particle physics and cosmology. At present neutrino physics is a
most vital domain of particle physics and cosmology and the existing open
questions are of crucial importance. We review the present status of the
subject, the main lessons that we have learnt so far and discuss the great
challenges that remain in this field.
