  Primary GCR interact with the Earth's atmosphere originating atmospheric
showers, thus giving rise to fluxes of secondary particles in the atmosphere.
Electromagnetic and hadronic interactions interplay in the production of these
particles, whose detection is performed by means of complementary techniques in
different energy ranges and at different depths in the atmosphere, down to the
Earth's surface.
  Monte Carlo codes are essential calculation tools which can describe the
complexity of the physics of these phenomena, thus allowing the analysis of
experimental data. However, these codes are affected by important
uncertainties, concerning, in particular, hadronic physics at high energy. In
this paper we shall report some results concerning inclusive particle fluxes
and atmospheric shower properties as obtained using the FLUKA transport and
interaction code. Some emphasis will also be given to the validation of the
physics models of FLUKA involved in these calculations.
