[en] Extensive air showers can only be interpreted by comparison of the measured observables with simulations, using a suitable model for the shower development in the atmosphere. Due to the complexity of the phenomenon, in the past the models were quite simple and results from air showers were to interpret qualitatively, at best. With a new generation of experiments more advanced detectors became operational and the need for detailed simulations grew. In recent years several new air shower programs were developed. Using the rapid increase of the computing power they have now reached a high level of sophistication. Particle transport, electromagnetic interactions and decay of unstable particles are treated in great detail. However the most crucial ingredient to all these programs is the modeling of hadronic interactions since it requires extrapolation in energy, primary mass and kinematics to regions where no accelerator data exist and where theoretical guidelines are only vague. As a consequence of the model uncertainties, experiments using different models occasionally reached very different conclusions on energy spectra and composition of the cosmic rays. CORSIKA is an extensive air shower simulation program that contains five different hadronic interaction models (VENUS, QGSJET, SIBYLL, HDPM, and DPMJET). It is widely used throughout the cosmic ray community and allows the study of model dependencies and the systematic effects of measurement and event reconstruction. Results for different models concerning inelastic cross-sections and particle production and their influence on air shower variables are discussed and the systematic uncertainties for air shower analyses are investigated. The capability of precise multi-parameter measurements to discriminate between the models is emphasised