  This review of atmospheric muons and neutrinos emphasizes the high energy
range relevant for backgrounds to high-energy neutrinos of astrophysical
origin. After a brief historical introduction, the main distinguishing features
of atmospheric $\nu_\mu$ and $\nu_e$ are discussed, along with the implications
of the muon charge ratio for the $\nu_\mu/\bar{\nu}_\mu$ ratio. Methods to
account for effects of the knee in the primary cosmic-ray spectrum and the
energy-dependence of hadronic interactions on the neutrino fluxes are discussed
and illustrated in the context of recent results from IceCube. A simple
numerical/analytic method is proposed for systematic investigation of
uncertainties in neutrino fluxes arising from uncertainties in the primary
cosmic-ray spectrum/composition and hadronic interactions.
