  Primary cosmic ray particles above energies of about 100 TeV are investigated
by large-area ground based detector installations, observing various components
of the development of extensive air-showers (EAS). By such an indirect access
to the primary cosmic ray spectrum a steepening of the power-law falls off at
around 3-5 PeV, known as the {\em knee} has been identified. Different
experimental techniques are used to deduce mass- and energy-sensitive
observables of EAS experiments. The experiments involve measurements of
secondary particle distributions at various observation levels and of muons
deep underground as well as of observables reconstructed by measuring the air
Cherenkov light emitted with the shower development in the atmosphere. Recently
methods for analysing multidimensional distributions get favoured since they
are able to take into account the correlations between different observables
and the influence of large intrinsic fluctuations of the air-shower
development. Additionally the use of a larger set of observables provides the
possibility to test simultaneously the consistency of the Monte-Carlo models
underlying the reconstruction procedures. By many experiments the existence of
the knee in the primary energy spectrum is confirmed and a tendency of an
increasing mean mass above the knee energy is indicated. Recent results show
that the knee originates from a decrease of the flux of light primary
particles, whereas heavy cosmic ray particles seem to miss a kink in the energy
range of 1 and 10 PeV.
