The discovery of cosmic rays

The work leading to the discovery of cosmic rays by Victor Hess on August 7, 1912 is reviewed. In particular the almost forgotten work of the Italian Dominico Pacini in 1909-10 is described. Hess was awarded the Nobel Prize in Physics 1936, shared with Carl Anderson for the discovery of the positron. The reason for the long delay is discussed as well as the nominations and discussions put forward in the Nobel Committee's report to the Royal Swedish Academy of Sciences.