The basic Standard Model of particle physics assumes that the neutrino is massless, although adding massive neutrinos to the basic framework is not difficult, and recent experiments suggest that the neutrino has a small although non-zero mass.

The strongest upper limits on the mass of the neutrino come from cosmology. The Big Bang model predicts that there is a fixed ratio between the number of neutrinos and the number of photons in the cosmic microwave background. If the total mass of all three types of neutrinos exceeded 50 electron volts (per neutrino), there would be so much mass in the universe that it would collapse. This limit can be circumvented by assuming that the neutrino is unstable; however, there are limits within the Standard Model that make this difficult.

However, it is now widely believed that the mass of the neutrino is non-zero. When one extends the Standard Model to include neutrino masses, one finds that massive neutrinos can change type whereas massless neutrinos cannot. This phenomenon, known as neutrino oscillation, explains why there are many fewer electron neutrinos observed from the sun and the upper atmosphere than expected, and has also been directly observed. One form of quantum gravity theory, due to Burkhard Heim, made neutrino mass predictions in the 1980s which are still consistent with experiment: ( 0.00381 ev, 0.00537 Mev, 0.010752 Mev) compared to current upper limit of (< 2.5 ev, .< 0.17 Mev, < 18.2 Mev) respectively.