Cosmic rays, fully ionized charged nuclei, have energy spectra spanning from GeV to $10^{20}$ eV. The region from $10^{14}$ to $10^{18}$ eV is particularly interesting, featuring the "knee" and "second knee" in the energy spectrum. These features are thought to be due to changes in the spectral index, with the knee at $4.5$ PeV and the second knee at $400$ PeV. Understanding these structures is key to understanding the origin of high-energy cosmic rays. Air shower experiments are used to study these features, with a simple Heitler model of air showers developed to explain the principles of these measurements. The energy spectrum of cosmic rays is generally a power law, but small variations in the spectral index are observed. The knee is attributed to a cutoff in the energy spectra of light elements, while the second knee is linked to the fall-off of heavier elements. The ankle, where the spectrum flattens again, is above $4$ EeV. Recent experiments, such as KASCADE, have provided insights into the composition and energy spectra of cosmic rays, showing that the all-particle spectrum is compatible with air shower experiments in the knee region. Above $10^8$ GeV, the galactic component is insufficient, suggesting an extragalactic contribution. The KASCADE-Grande experiment aims to extend measurements to $10^{18}$ eV, providing information on the mass composition in the second knee region. The study highlights the importance of understanding cosmic ray acceleration, propagation, and interactions in the atmosphere.Cosmic rays, fully ionized charged nuclei, have energy spectra spanning from GeV to $10^{20}$ eV. The region from $10^{14}$ to $10^{18}$ eV is particularly interesting, featuring the "knee" and "second knee" in the energy spectrum. These features are thought to be due to changes in the spectral index, with the knee at $4.5$ PeV and the second knee at $400$ PeV. Understanding these structures is key to understanding the origin of high-energy cosmic rays. Air shower experiments are used to study these features, with a simple Heitler model of air showers developed to explain the principles of these measurements. The energy spectrum of cosmic rays is generally a power law, but small variations in the spectral index are observed. The knee is attributed to a cutoff in the energy spectra of light elements, while the second knee is linked to the fall-off of heavier elements. The ankle, where the spectrum flattens again, is above $4$ EeV. Recent experiments, such as KASCADE, have provided insights into the composition and energy spectra of cosmic rays, showing that the all-particle spectrum is compatible with air shower experiments in the knee region. Above $10^8$ GeV, the galactic component is insufficient, suggesting an extragalactic contribution. The KASCADE-Grande experiment aims to extend measurements to $10^{18}$ eV, providing information on the mass composition in the second knee region. The study highlights the importance of understanding cosmic ray acceleration, propagation, and interactions in the atmosphere.