Spectrum and composition of cosmic rays at energies of $10^{15}$-$10^{18}$ eV

EThe energy spectrum and mass composition of primary cosmic rays at energies of $10^{15}$-$10^{18}$ eV are considered on the basis of experimental data from the EAS MSU array, which detects showers by charged particles, and the Tunka-25 array, which uses shower-produced Cherenkov radiation to detect EASs. A change in composition due to an increase in the fraction of heavy nuclei is shown to occur starting from the energy of the knee in the primary energy spectrum and up to $10^{17}$ eV. At energies above $10^{17}$ eV, an additional component that is unrelated to the bulk of the Galactic cosmic rays generated by shock waves in supernova explosions may appear in the energy spectrum. In this case, the fraction of heavy nuclei begins to decrease and the composition is again enriched by light nuclei. A brief description of the new Tunka-133 array with an area of about 1 km$^2$ is given.