  Production of muons and neutrinos in cosmic ray interactions with the
atmosphere has been investigated with Monte Carlo models for hadronic
interactions. The resulting conventional muon and neutrino fluxes (from $\pi$
and $K$ decays) agree well with earlier calculations, whereas our prompt fluxes
from charm decays are significantly lower than earlier estimates. Charm
production is mainly considered as a well defined perturbative QCD process, but
we also investigate a hypothetical non-perturbative intrinsic charm component
in the proton. The lower charm rate implies better prospects for detecting very
high energy neutrinos from cosmic sources.
