The physics of cosmic rays was intensively developed during the last century. The subject of ultra high energy cosmic rays has received special attention during the last partly due to the controversy between the results of the HiRes and AGASA Observatories and the puzzle caused by super‐GZK observed events. In this article we review the subject of cosmic rays, first from a historical point of view, then we give examples of methods of measuring cosmic ray properties, including those used in HiRes and AGASA.. © 2003 American Institute of Physics