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 the current situation, 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 be discussed in this context. Part of this will be based on simulation with CRPropa 2.0, our new public multimessenger propagation code.
