  Detailed investigations of extensive air showers have been performed with the
data measured by the KASCADE experiment. The results allow to evaluate hadronic
interaction models, used in simulations to interpret air shower data. The
all-particle spectrum of cosmic rays and their mass composition, as well as
individual spectra for groups of elements have been reconstructed. The results
suggest, the knee in the all-particle cosmic--ray energy spectrum is caused by
a rigidity-dependent cut-off of individual element groups.
