Neutrinos perhaps best illustrate the close relationship that exists between cosmology and elementary particle physics. Cosmological and astrophysical observations can be used to constrain neutrino properties. The abundance of the4He probably limits the number of light (<1 MeV) neutrino species to be less than or equal to 4. An unstable neutrino with lifetime &agr;m&ngr;−5is restricted to be less massive than &angupr;220 eV or more massive than &angupr;10‐100 MeV. Neutrinos more massive than a few eV have important cosmological consequences: if the sum of neutrino masses ≳3.5 eV the universe is neutrino‐dominated, and if it is ≳100 eV the universe is closed. The role that neutrinos play in the development of structure in the universe (galaxies, clusters, etc.) is also discussed.