  This thesis presents an original model for the FIR emission in spirals,
developed from an existing radiative transfer code (Bianchi, Ferrara &
Giovanardi, 1996, ApJ, 465, 127). The model's main features are: a complete
treatment of multiple scattering within geometries appropriate for spirals; a
full coverage of spectral range of stellar emission; the use of empirically
determined dust properties (some of which are derived in the present work); the
production of maps of the dust temperature distribution, together with
simulated optical, FIR and sub-millimetre images. The model has been applied to
observations of stellar and dust emission in the galaxy NGC 6946.
  It is found that optically thick models (central face-on optrical depth tau_V
\~ 5) are necessary to explain the observed FIR output. For such models, almost
30-40% of the intrinsic starlight is absorbed. The observed ratio of FIR and
optical scalelengths can be explained if the dust distribution is more extended
than the stellar. However, because of the less steep gradients of optical
emission in optically thick cases, a very extended dust distribution is needed
(alpha_d ~ 3 alpha_s). The distribution of atomic gas in NGC 6946 has a similar
extent.
  I discuss the approximations in the modelling (mainly the use of smooth
distributions against the observed clumpiness of the interstellar medium) and
the implications of the results.
