  The description of the tempo-spatial evolution of the composition of cosmic
gas on galactic scales is called 'modelling galactic chemical evolution'. It
aims to use knowledge about sources of nucleosynthesis and how they change the
composition of interstellar gas, following the formation of stars and the
ejection of products from nuclear fusion during their evolution and terminating
explosions. Sources of nucleosynthesis are diverse: Stars with hydrostatic
nuclear burning eject some of the products, and core-collapse supernovae add
ejecta. Binary interactions lead to sources such as thermonuclear supernovae
and kilonovae. Tracing ejecta from sources, with their different frequencies
and environments, through the interstellar medium and successive star formation
cycles is the goal of model descriptions.
  A variety of formalisms exist, from analytical through semi-analytical,
numerical or stochastic approaches, gradually making descriptions of
compositional evolution of cosmic matter more realistic, teaching us about the
astrophysical processes involved in this complex aspect of our universe.
  Radioactive isotopes add important ingredients to such modelling: The
intrinsic clock of the radioactive decay process adds a new aspect to the
modelling algorithms that leads to different constraints on the important
unknowns of star formation activity and interstellar transports. Several
prominent examples illustrate how modelling the abundances of radioactive
isotopes and their evolution have resulted in new lessons; among these are the
galaxy-wide distribution of 26Al and 60Fe, the radioactive components of cosmic
rays, the interpretations of terrestrial deposits of 60Fe and 244Pu, and the
radioactive-decay daughter isotopes that were found in meteorites and
characterise the birth environment of our solar system.
