"The Universe is in fact observed not only through the different windows of the electromagnetic spectrum, but also through other cosmic messengers, i.e. through cosmic rays (CRs), neutrinos and gravitational waves (GWs). In general, gamma rays are the perfect companions for multi-messenger astronomy ... gamma-ray production is intimately related to the production of CRs. The latter are charged particles, mainly protons, whose energy spectrum covers a very wide range in energy and flux. Many questions regarding CRs are still open, especially looking at the most energetic ones above 1015 eV (1 PeV). The CR spectrum is approximately described by a power law: dN/dE ∼ E−Γ , where Γ is the spectral index. Γ is not constant, indicating a change in the properties of CRs, like their acceleration sites and chemical composition. For energies around ∼ 4 × 1015 eV, the flux starts to decrease more steeply: Γ changes from about 2.7 to about 3. This feature, marked with the term knee, is thought to indicate the maximum acceleration energy of Galactic sources ..."