Understanding the structure of the gaseous component of disk galaxies is an important step for unraveling their on-going evolution. In particular, extraplanar gas in galaxies is a potentially-important probe of the effects of kinetic and radiative feedback from massive stars to the gas in galaxies, as well as of the role that the accretion of metal-poor gas plays in the evolution of modern galaxies. Most studies of extraplanar matter have focused on the gas content of the interstellar thick disks and halos of galaxies. However, the processes that transport matter from the thin disks of galaxies into the thick disks and extended halos will act on both gas and interstellar dust grains. The presence of extraplanar dust in galaxies can strongly affect the thermal balance of the gas, and it will definitely affect an observer's view of a galaxy through its impact on the transfer of radiation.
The presence of extraplanar dust in the canonical edge-on galaxy NGC 891 was noted many years ago (Sandage 1961; Dettmar 1990; Keppel et al. 1991). However, an analysis of the implications of the extensive web of dusty extraplanar material in this galaxy did not occur until recently (Howk & Savage 1997, 2000). In the last few years, extraplanar dust has been recognized and studied in many edge-on systems (Sofue, Wakamatsu, & Malin 1994; Howk & Savage 1999; Alton et al. 2000; Rossa & Dettmar 2003; Thompson, Howk, & Savage 2004), and we now know that the presence of extraplanar dust and gas are indeed coupled.
In this contribution, I will concentrate on the physical interpretation of the observed extraplanar dust and its implications for the nature of the interstellar thick disks of spiral galaxies. In particular, I will summarize some of the evidence that material identified with extraplanar dust may represent a dense, cold medium in the thick disks of galaxies.