  The opacity of a spiral disk due to dust absorption influences every
measurement we make of it in the UV and optical. Two separate techniques
directly measure the total absorption by dust in the disk: calibrated distant
galaxy counts and overlapping galaxy pairs. The main results from both so far
are a semi-transparent disk with more opaque arms, and a relation between
surface brightness and disk opacity. In the Spitzer era, SED models of spiral
disks add a new perspective on the role of dust in spiral disks. Combined with
the overall opacity from galaxy counts, they yield a typical optical depth of
the dusty ISM clouds: 0.4 that implies a size of $\sim$ 60 pc. Work on galaxy
counts is currently ongoing on the ACS fields of M51, M101 and M81. Occulting
galaxies offer the possibility of probing the history of disk opacity from
higher redshift pairs. Evolution in disk opacity could influence distance
measurements (SN1a, Tully-Fisher relation). Here, we present first results from
spectroscopically selected occulting pairs in the SDSS. The redshift range for
this sample is limited, but does offer a first insight into disk opacity
evolution as well as a reference for higher redshift measurements. Spiral disk
opacity has not undergone significant evolution since z=0.2. HST imaging would
help disentangle the effects of spiral arms in these pairs. Many more
mixed-morphology types are being identified in SDSS by the GalaxyZoo project.
The occulting galaxy technique can be pushed to a redshift of 1 using many
pairs identified in the imaging campaigns with HST (DEEP2, GEMS, GOODS,
COSMOS).
