  Thanks to compelling evidence that neutrinos can change flavor, we now know
that they have nonzero masses, and that leptons mix. In these lectures, we
explain the physics of neutrino flavor change, both in vacuum and in matter.
Then, we describe what the flavor-change data have taught us about neutrinos.
Finally, we consider some of the questions raised by the discovery of neutrino
mass, explaining why these questions are so interesting, and how they might be
answered experimentally,
