  This article summarizes the possible roles of neutrinos in cosmology, from
the first three minutes onward. The fact that primordial neutrinos are about as
numerous as the photons of the cosmological background radiation means that
neutrino (or ``hot'') dark matter can have a significant impact upon how matter
is distributed in the universe. Over the past two decades, the liklihood of
this has risen and fallen as more and more data has become available from
laboratory experiments and the latest telescopes. Hot dark matter models were
studied in the late 1970s and early 1980s, and there was considerable interest
in cold plus hot dark matter models for a few years in the mid-1990s. We now
know from Super-Kamiokande and Sudbury Neutrino Observatory data that neutrinos
have mass, and that the total neutrino contribution to the cosmological density
must be at least as large as $\Omega_\nu \sim 0.001$, a little less than that
of the visible stars in the universe. But the upper limit on the hot dark
matter cosmological density is $\Omega_\nu < 0.1$; correspondingly, the sum of
the masses of all three active neutrino species is not more than about 5 eV.
