The Standard Model predicts that the neutrinos are massless and do not mix. Generic extensions of the Standard Model predict that neutrinos are massive (but, very likely, much lighter than the charged fermions). Therefore, the search for neutrino masses and mixing tests the Standard Model and probes new phasics.
Measurements of various features of the fluxes of atmospheric, solar and, more recently, reactor neutrinos have provided evidence for neutrino oscillations and therefore for neutrino masses and mixing.
These results have significant theoretical implications: new physics exists, and its scale can be estimated. There are interesting lessons for grand unified theories and for models of extra dimensions. The measured neutrino flavor parameters pose a challenge to flavor models.
