Hadronic Interactions at Cosmic Ray Energies

General physics of very high energy hadronic interactions is discussed. Special attention is paid to the contribution of semihard processes to the interaction dynamics and to the role of parton shadowing and parton density saturation. In particular, the implementation of non-linear interaction effects in the QGSJET-II model is discussed in detail. The predictions of the model are compared to selected accelerator data, including ones of the RHIC collider, and the relation to the calculated extensive air shower characteristics is discussed. Finally, the potential of accelerator and cosmic ray experiments for constraining model predictions is analyzed.