Cosmic radioactivity represents a cross-disciplinary theme and an interesting alternate viewpoint on cosmic nuclear astrophysics. Radioactive isotopes and their decay provide unique messages from sites of cosmic nucleosynthesis, as the decay is mediated by weak interaction physics and independent of environmental conditions. The radioactive clock of various isotopes traces stellar mixing processes and the process of solidification of bodies when the solar system was formed. Isotopic abundances directly reflect the conditions of their formation in the nucleosynthesis site, which is unobservable otherwise. Measurements range from meteorites and their included stardust grain compositions through to cosmic rays and electromagnetic radiation from infrared to gamma ray wavelengths. Thus, various astronomical disciplines with their different messengers of cosmic nucleosynthesis as seen through unstable, decaying isotopes are linked to the physics of nuclear reactions, and to theories and models of the variety of cosmic nucleosynthesis sites and of cosmic isotopic evolution.