This book introduces you to the physics of cosmic rays, charged particles which reach us from known--and maybe unknown--sources in the cosmos. Starting from a brief history of this fascinating field, it reviews what we know about the creation of elements in the Big Bang and inside stars. It explains cosmic accelerators reaching fabulous energies. It follows the life cycle of cosmic rays all the way from their sources to detection near, on or below Earth. The central three chapters cover what we know about them at the level of the solar system, the Milky Way, and the Universe at large. Up-to-date experimental results are presented in detail, showing how they are obtained and interpreted. The book provides an accessible overview of this lively and diversified research field. It will be of interest to undergraduate physics students beginning their studies on astronomy, cosmology, and particle physics. It is also accessible to the general public by concentrating mathematical and technical detail into Focus Boxes.