We review the prehistory and early history of cosmic ray studies, concentrating on the period 1900–1927. Following the discoveries of the electron and radioactivity just before the turn of the century, the old problem of leakage of charge from a conductor in air was investigated in terms of the new concept of ionization and ionizing radiation, part of which was found to be highly penetrating and to be of extraterrestrial origin. At first supposed to consist only of ultrahigh energy gamma rays, the cosmic ray primaries are now known to be mainly charged particles. The modern period of cosmic ray research began in 1927, when individual particles were studied by cloud‐chamber and coincidence counting techniques.