  The Pierre Auger Observatory in Argentina provides the largest data sample of
the cosmic ray events with energy above 10$^{18}$ eV. These high energy events
can be used to test our understanding of the hadronic interactions at energies
beyond the reach of colliders and to probe the basic properties of these
interactions such as the inelastic cross-section of proton-air collisions. The
combination of an array of surface detectors and the fluorescence telescopes of
the Pierre Auger Observatory reduces significantly the dependency of the shower
energy estimation on MC simulations. Despite that, the interpretation of mass
sensitive quantities such as the shower maximum in terms of chemical
composition of cosmic rays still depends on the hadronic interaction models.
This contribution describes the main results of the observatory concerning the
chemical composition of the cosmic rays and focuses on the problem of muon
deficit in hadronic interaction models and on the estimation of proton-air
cross-section from air-shower data.
