  Cosmic rays fill up the entire volume of galaxies, providing an important
source of heating and ionisation of the interstellar medium, and may play a
significant role in the regulation of star formation and galactic evolution.
Diffuse emissions from radio to high-energy gamma rays (> 100 MeV) arising from
various interactions between cosmic rays and the interstellar medium,
interstellar radiation field, and magnetic field, are currently the best way to
trace the intensities and spectra of cosmic rays in the Milky Way and other
galaxies. In this contribution, I describe our recent work to model the full
spectral energy distribution of galaxies like the Milky Way from radio to
gamma-ray energies. The application to other galaxies, in particular the
Magellanic Clouds and M31 that are detected in high-energy gamma-rays by the
Fermi-LAT, is also discussed.
