Cosmic rays have been observed up to several 10^20 eV through the showers of secondary particles they induce in the atmosphere. Their existence poses formidable challenges and exciting prospects at the same time: Their origin and sources have not been identified yet, but they already allow to test physics at center of mass energies unattained in the laboratory, albeit in a rather indirect way. We will give an overview over possible sources and acceleration mechanisms, open questions, and future prospects, including the role of secondary gamma-rays and neutrinos produced in interactions of charged primary cosmic rays.
Possible origins of the two tentative extraterrestrial neutrino detections
by IceCube will also briefly be discussed in this context.