If galaxies consisted only of stars, and some early-type systems in general and dwarf spheroidal galaxies in particular fit this prescription, then the calculation of the SED in principle is straightforward. The emergent luminosity at any wavelength simply is the sum over all the luminosities of all the stars in the system. This can be calculated, of course provided that one has a complete understanding of stellar populations, which remains a non-trivial issue. Most galaxies, however, also contain an interstellar medium (ISM). The ISM absorbs, scatters and reprocesses the radiation and relativistic particles from sources within galaxies, primarily stars and AGN. That the ISM is neither isotropic nor homogeneous adds to the challenge of how to properly account for its influence on the luminosity emerging from galaxies.