The origin of cosmic rays (CR) has been intriguing scientists since 1912 when Victor Hess carried out his famous balloon flight to measure the ionization rate in the upper atmosphere. The total energy density of CR (nuclei, protons, pions, electrons, and gamma-rays) turns out to be comparable to that of the interstellar radiation and magnetic fields, and turbulent motions of the interstellar gas. This makes CR one of the essential factors determining the dynamics and processes in the interstellar medium (ISM).