  ISO observations at 200 micron have modified our view of the dust component
in spiral galaxies. For a sample of seven resolved spirals we have retrieved a
mean temperature of 20K, about 10K lower than previous estimates based on IRAS
data at shorter wavelengths. Because of the steep dependence of far-infrared
emission on the dust temperature, the dust masses inferred from ISO fluxes are
a factor of 10 higher than those derived from IRAS data only, leading to
gas-to-dust ratios close to the value observed in the Galaxy. The scale-length
of the 200 micron emission is larger than for the IRAS 100 micron emission,
with colder dust at larger distances from the galactic centre, as expected if
the interstellar radiation field is the main source of dust heating. The 200
micron scale-length is also larger than the optical, for all the galaxies in
the sample. This suggests that the dust distribution is more extended than that
of the stars.A model of the dust heating is needed to derive the parameters of
the dust distribution from the FIR emission. Therefore, we have adapted an
existing radiative transfer code to deal with dust emission. Simulated maps of
the temperature distribution within the dust disk and of the dust emission at
any wavelength can be produced. The stellar spectral energy distribution is
derived from observations in the ultraviolet, optical and near infrared. The
parameters of the dust distribution (scale-lengths and optical depth) are
chosen to reproduce the observed characteristics of the FIR emission, i.e. the
shape of the spectrum, the flux and the spatial distribution. We describe the
application of the model to one of the galaxies in the sample, NGC 6946.
