  Radioactive decay of unstable atomic nuclei leads to liberation of nuclear
binding energy in the forms of gamma-ray photons and secondary particles
(electrons, positrons); their energy then energises surrounding matter.
Unstable nuclei are formed in nuclear reactions, which can occur either in hot
and dense extremes of stellar interiors or explosions, or from cosmic-ray
collisions. In high-energy astronomy, direct observations of characteristic
gamma-ray lines from the decay of radioactive isotopes are important tools to
study the process of cosmic nucleosynthesis and its sources, as well as tracing
the flows of ejecta from such sources of nucleosynthesis. These observations
provide a valuable complement to indirect observations of radioactive energy
deposits, such as the measurement of supernova light in the optical. Here we
present basics of radioactive decay in astrophysical context, and how gamma-ray
lines reveal details about stellar interiors, about explosions on stellar
surfaces or of entire stars, and about the interstellar-medium processes that
direct the flow and cooling of nucleosynthesis ashes once having left their
sources. We address radioisotopes such as $^{56}$Ni, $^{44}$Ti, $^{26}$Al,
$^{60}$Fe, $^{22}$Na, $^{7}$Be, and also how characteristic gamma-ray emission
from the annihilation of positrons is connected to these.
