  The energies of cosmic rays, fully ionized charged nuclei, extend over a wide
range up to 10^20 eV. A particularly interesting energy region spans from
  10^14 to 10^18 eV, where the all-particle energy spectrum exhibits two
interesting structures, the 'knee' and the 'second knee'. An explanation of
these features is thought to be an important step in understanding of the
origin of the high-energy particles. Recent results of air shower experiments
in this region are discussed. Special attention is drawn to explain the
principle of air shower measurements - a simple Heitler model of (hadronic) air
showers is developed.
