Cosmic rays are not rays. They are high energy particles arriving from outside the atmosphere, produced by the Sun and a number of different types of high energy astronomical sources. I first explain how, in the early twentieth century Victor Hess in a high altititude balloon showed that they do not have a terrrestrial origin, and how Robert Millikan gave them the misleading name of cosmic rays, which has persisted. You will then read about their composition and how it has been determined with the AMS experiment on the International Space Station. Between 1930 and 1960 cosmic ray experiments were major players in particle physics. The positron, the pi meson (or pion) the kaon, and the ʌ hyperon were all discovered in this way. One feature of cosmic rays is their energies; the highest energy cosmic rays have 30 million times more energy than the particles in the CERN particle accelerator. These particles, as well as the highest energy gamma-rays, are detected via the showers of secondary particles they produce when they enter the upper atmosphere, using detectors on very large telescopes or distributed on the ground, which are described in this chapter. Their detection gives us information about supernovae within the galaxy, as well as the powerful sources around the central black holes in other galaxies, and the mergers of dense stellar objects such as back holes. We look forward to a future in which information from cosmic rays, neutrinos, and gravitational waves will combine with electromagnetic detection to probe the highest energy most distant processes in the universe.