Origin of very high- and ultra-high-energy cosmic rays

While there is some level of consensus on a Galactic origin of cosmic rays up to the knee ( E k ∼ 3 × 10 15   eV ) and on an extragalactic origin of cosmic rays with energy above ∼1019 eV, the debate on the genesis of cosmic rays in the intermediate energy region has received much less attention, mainly because of the ambiguity intrinsic in defining such a region. The energy range between 1017 eV and ∼1019 eV is likely to be the place where the transition from Galactic to extragalactic cosmic rays takes place. Hence the origin of these particles, though being of the highest importance from the physics point of view, it is also one of the most difficult aspects to investigate. Here I will illustrate some ideas concerning the sites of acceleration of these particles and the questions that their investigation may help answer, including the origin of ultra-high-energy cosmic rays.