Origin(s) of Cosmic Rays

The problem of the origin of Cosmic Rays is now over a century old and while there has been substantial progress, especially in the last decade, there are still open questions. The question of "origin" is open to at least three possible interpretations depending on whether one follows the energy powering the accelerator, the matter being accelerated, or the physics of the acceleration process; these approaches are reviewed in turn. Supernova remnants remain by far the most plausible candidates as dominant sources for the bulk of the Galactic cosmic rays, but contributions from other source populations remain possible. The transition at higher energies from Galactic to extra-galactic populations remains obscure.