The article by Jörg R. Hörandel discusses the energy spectrum of cosmic rays, focusing on the energy range from 10^14 to 10^18 eV, where two significant structures, the "knee" and the "second knee," are observed. The knee occurs at around 4.5 PeV, where the power-law spectral index changes from γ = -2.7 to γ ≈ -3.1, and the second knee appears at 400 PeV, with a further steepening of the spectrum to γ ≈ -3.3. These structures are crucial for understanding the origin of high-energy cosmic rays. The article reviews recent results from air shower experiments, which are used to study these structures. A simple Heitler model of air showers is developed to explain the principles of air shower measurements. The model describes the electromagnetic and hadronic components of air showers, including the number of electrons and muons at shower maximum, and the depth of the shower maximum. The article also discusses the experimental techniques used to measure the flux of individual elements and the composition of cosmic rays, highlighting the importance of the KASCADE experiment in advancing our understanding of the energy spectra and composition of cosmic rays. The conclusion emphasizes the need for further experiments to better understand the origin of high-energy cosmic rays, particularly in the energy range above the knee.The article by Jörg R. Hörandel discusses the energy spectrum of cosmic rays, focusing on the energy range from 10^14 to 10^18 eV, where two significant structures, the "knee" and the "second knee," are observed. The knee occurs at around 4.5 PeV, where the power-law spectral index changes from γ = -2.7 to γ ≈ -3.1, and the second knee appears at 400 PeV, with a further steepening of the spectrum to γ ≈ -3.3. These structures are crucial for understanding the origin of high-energy cosmic rays. The article reviews recent results from air shower experiments, which are used to study these structures. A simple Heitler model of air showers is developed to explain the principles of air shower measurements. The model describes the electromagnetic and hadronic components of air showers, including the number of electrons and muons at shower maximum, and the depth of the shower maximum. The article also discusses the experimental techniques used to measure the flux of individual elements and the composition of cosmic rays, highlighting the importance of the KASCADE experiment in advancing our understanding of the energy spectra and composition of cosmic rays. The conclusion emphasizes the need for further experiments to better understand the origin of high-energy cosmic rays, particularly in the energy range above the knee.