  We review some of the recent progress in our knowledge about high-energy
cosmic rays, with an emphasis on the interpretation of the different
observational results. We discuss the effects that are relevant to shape the
cosmic ray spectrum and the explanations proposed to account for its features
and for the observed changes in composition. The physics of air-showers is
summarized and we also present the results obtained on the proton-air cross
section and on the muon content of the showers. We discuss the cosmic ray
propagation through magnetic fields, the effects of diffusion and of magnetic
lensing, the cosmic ray interactions with background radiation fields and the
production of secondary neutrinos and photons. We also consider the cosmic ray
anisotropies, both at large and small angular scales, presenting the results
obtained from the TeV up to the highest energies and discuss the models
proposed to explain their origin.
