  Atmospheric neutrinos produced by cosmic-ray interactions around the globe
provide a beam for the study of neutrino properties. They are also a background
in searches for neutrinos of astrophysical origin. Both aspects are addressed
in this chapter, which begins with a brief introduction on neutrino
oscillations in relation to the spectrum of atmospheric neutrinos. Section 2
describes the cascade equation for hadrons in the atmosphere and the main
features of atmospheric leptons from their decays. Next, uncertainties in the
fluxes that arise from limited knowledge of the primary spectrum and of
particle production are discussed. The final section covers aspects specific to
neutrino telescopes.
