  Modelling the UV/optical - infrared/submm SEDs of spiral galaxies observed
with Herschel will be an essential tool to quantitatively interpret these
observations in terms of the present and past star-formation activity of these
systems. In this lecture we describe the SED modelling technique we have
developed, its applications and tests of its predictions. We show that both the
panchromatic SED modelling of individual galaxies and the B-band
attenuation-inclination relation of large statistical samples suggest that
spiral galaxies in the nearby Universe behave as optically thick systems in
their global properties and large-scale distribution of light (central face-on
B-band opacity of approx. 4). However disk galaxies are very inhomogeneous
systems, having both optically thick components (e.g. spiral arms), and
optically thin components (e.g. the interarm regions), the latter making
galaxies transparent to background galaxies.
