  I recall the place of neutrinos in the electroweak theory and summarize what
we know about neutrino mass and flavor change. I next review the essential
characteristics expected for relic neutrinos and survey what we can say about
the neutrino contribution to the dark matter of the Universe. Then I discuss
the standard-model interactions of ultrahigh-energy neutrinos, paying attention
to the consequences of neutrino oscillations, and illustrate a few topics of
interest to neutrino observatories. I conclude with short comments on the
remote possibility of detecting relic neutrinos through annihilations of
ultrahigh-energy neutrinos at the $Z$ resonance.
