  We know that neutrino mass and mixing provide a window to physics beyond the
Standard Model. Now this window is open, at least partly. And the questions
are: what do we see, which kind of new physics, and how far "beyond"? I
summarize the present knowledge of neutrino mass and mixing, and then formulate
the main open questions. Following the bottom-up approach, properties of the
neutrino mass matrix are considered. Then different possible ways to uncover
the underlying physics are discussed. Some results along the line of: see-saw,
GUT and SUSY GUT are reviewed.
