While there is some level of consensus on a Galactic origin of cosmic rays up
to the knee (Ek∼3×1015 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 \underline{ultra} high energy cosmic rays.Comment: Solicited Review Paper to appear in 'Comptes Rendus Physique
