1896 Henri Becquerel, experimenting on supposed production of X-rays by fluorescent materials, discovered a form of radiation coming from uranium, independent of any fluorescence. Read his reports to the French Academy of Science in his own words. His discovery of radioactivity earned his Nobel prize in 1903, shared with Pierre and Marie Curie.
1898 Marie Curie and G.C.Schmidt independently discovered that thorium and its compounds are radioactive. M. Curie found higher than expected activity in some minerals containing uranium and thorium. Read Marie Curie's report.
1898 Marie Curie and Pierre Curie found, associated with bismuth in mineral ores, a new radioactive element, polonium, and, associated with barium in ores, another new radioactive element, radium. Their 1903 Nobel prize for Physics was shared with Becquerel. Marie Curie was later awarded the 1911 Nobel prize, this time for Chemistry, "in recognition of her services to the advancement of chemistry by the discovery of the elements radium and polonium, by the isolation of radium and the study of the nature and compounds of this remarkable element".
By 1906, only 10 years after Becquerel's discovery, 20 different radioactive substances had been discovered, their differences being detected by the nature of the radiations emitted.