  The results from a large field Far-Infrared (FIR) and sub-millimeter (sub-mm)
survey of our neighbor galaxy M31 are presented. We have obtained Herschel
images of a ~5.5x2.5 degree area centered on Andromeda. Using 21 cm atomic
hydrogen maps, we are able to disentangle genuine emission from M31 from that
for foreground Galactic cirrus, allowing us to recognize dusty structures out
to ~31 kpc from the center. We first characterize the FIR and sub-mm morphology
and then, by de-projecting Herschel maps and running an ad--hoc source
extraction algorithm, we reconstruct the intrinsic morphology and the spatial
distribution of the molecular complexes. Finally, we study the spatially
resolved properties of the dust (temperature, emissivity, mass, etc.), by means
of a pixel-by-pixel SED fitting approach.
