  Continuum gamma-ray emission produced by interactions of cosmic rays with
interstellar matter and radiation fields is a probe of non-thermal particle
populations in galaxies. After decades of continuous improvements in
experimental techniques and an ever-increasing sky and energy coverage,
gamma-ray observations reveal in unprecedented detail the properties of
galactic cosmic rays. A variety of scales and environments are now accessible
to us, from the local interstellar medium near the Sun and the vicinity of
cosmic-ray accelerators, out to the Milky Way at large and beyond, with a
growing number of gamma-ray emitting star-forming galaxies. Gamma-ray
observations have been pushing forward our understanding of the life cycle of
cosmic rays in galaxies and, combined with advances in related domains, they
have been challenging standard assumptions in the field and have spurred new
developments in modelling approaches and data analysis methods. We provide a
review of the status of the subject and discuss perspectives on future
progress.
