It is more than a century now since the phenomenon of radioactivity has been discovered. As noticed by D. D. Clayton in his introduction to Chap. 2, E. Rutherford was the first to point out that radioactive elements were produced in the Cosmos: In 1929 he had suggested that the long lived actinides uranium and thorium were synthesised in the Sun about 100 Myr ago and were somehow transported to Earth. It took another 20 years before another scientist suggested that radioactivity occurs now in the Universe and has observable effects: in 1950, L. Borst noticed that the exponential decrease of the luminosity of Type I supernovae (with a characteristic timescale of 55 days) is a strong indication for a radioactive source. But he misidentified that source with 7Be-decay. Two years later, P. Merrill identified absorption lines of short-lived technetium in the spectra of S-stars, a clear indication that radioactivity (and nucleosynthesis) occurs in the Universe today.