Abstract : Cosmic rays play a very special role in astrophysics and astroparticle physics, not only because they are involved in most of the energetic processes taking place in the universe, but because their very existence and the continuity of their energy spectrum (from thermal energies up to over 1020 eV) raise fruitful and unexpectedly diverse questions. The source of these energetic particles travelling across the cosmos remains largely unknown, and decades of research have not allowed one to fully undestand the link between those that are seen to radiate throughout the electromagnetic spectrum in the various astronomical sources and those that are detected in the vicinity of Earth, carrying precious information for physics in general. We propose a global and multidisciplinary approach to cosmic rays, emphasizing the necessity of considering together their three spectral dimensions: energy distribution (spectrum), angular distribution (anisotropies) and mass distribution (composition). We analyse various aspects of the problematics — theoretical, phenomenological and experimental — with some focus on ultra-high-energy cosmic rays, in relation with the Pierre Auger Observatory. But we also try to explicate the links between the different ranges of energy and to emphasize the importance of the galactic/extragalactic transition. Numerous constraints are also provided by non-thermal astronomy and nuclear astrophysics, of which we discuss some aspects that are directly related to energetic particles, to their acceleration and their propagation in the cosmos.