  The atmospheric neutrino beam simultaneously spans a range of pathlengths
from ten to ten thousand kilometers, which correspond respectively to downward-
and upward-going neutrinos. As with any neutrino oscillation experiment, also
in this case the interpretation of the data depends on a detailed knowledge of
the neutrino beam. The ingredients are the primary spectrum of cosmic-ray
nucleons, the geomagnetic fields in which the charged particles propagate and
the properties of interactions of hadrons in the atmosphere. In this talk I
review the status of calculations in light of the recent evidence for neutrino
oscillations from Super-Kamiokande (Y. Fukuda et al., Phys. Rev. Letters 81
(1998) 1562).
