  When high-energy cosmic rays (gamma's, protons, or heavy nuclei) impinge onto
the Earth's atmosphere, they interact at high altitude with the air nuclei as
targets. By repeated interaction of the secondaries an `extensive air shower'
(EAS) is generated with huge particle numbers in the maximum of the shower
development. Such cascades are quantitatively simulated by the Monte Carlo
computer program CORSIKA. The most important uncertainties in simulations arise
from modeling of high-energy hadronic interactions: a) The inelastic hadron-air
cross sections. b) The energies occurring in EAS may extend far above the
energies available in man-made accelerators, and when extrapolating towards
higher energies one has to rely on theoretical guidelines. c) In collider
experiments which are used to adjust the interaction models the very forward
particles are not accessible, but just those particles carry most of the
hadronic energy, and in the EAS development they transport a large energy
fraction down into the atmosphere. CORSIKA is coupled alternatively with 6
high-energy hadronic interaction codes (DPMJET, HDPM, neXus, QGSJET, SIBYLL,
VENUS). The influence of those interaction models on observables of simulated
EAS is discussed.
