  Cosmological dark matter in the form of neutrinos with masses of up to a few
electron volts is known as hot dark matter. After an historical review of the
subject, this article considers constraints on hot dark matter from current
data on neutrino oscillations and on cosmology. The atmospheric neutrino
oscillation data imply a lower limit on the HDM contribution to the
cosmological density $\Omega_\nu \gsim 0.001$. The possible improvement of
low-$\Omega_m$ flat ($\Lambda$CDM) cosmological models with the addition of
light neutrinos appears to be rather limited, but$\Lambda$CDM models with
$\Omega_\nu \lsim 0.1$ may be consistent with presently available data. Data
expected soon may permit detection of such a hot dark matter contribution, or
alternatively provide stronger upper limits on $\Omega_\nu$ and neutrino
masses.
