  The interstellar thick disks of galaxies contain not only gas, but
significant quantities of dust. Most of our knowledge of extraplanar dust in
disk galaxies comes from direct broadband optical imaging of these systems,
wherein the dust is identified due to the irregular extinction it produces
against the thick disk and bulge stars. This observational technique is
sensitive to only the most dense material, and we argue much of the material
identified in this way traces a cold phase of the interstellar thick disks in
galaxies. The presence of a cold, dense phase likely implies the interstellar
pressures in the thick disks of spiral galaxies can be quite high. This dense
phase of the interstellar medium may also fueling thick disk star formation,
and H-alpha observations are now revealing H II regions around newly-formed OB
stars associations in several galaxies. We argue that the large quantities of
dust and the morphologies of the structures traced by the dust imply that much
of the extraplanar material in disk galaxies must have been expelled from the
underlying thin disk.
