To constrain the dust size distribution and the size and geometry of the dust torus, various models have been proposed to explain the observed IR emission spectral energy distribution (SED) of AGNs, radiated by the circumnuclear dust heated by the AGN illumination. These models assume a wide range of torus geometries: uniform density annular (cylindrical) rings of a few pc with an extremely large optical depth UV > 1000 (Pier & Krolik 1992, 1993), optically thick plane parallel slabs of a few thousand pc (Laor & Draine 1993), extended tori of hundreds of pc (Granato & Danese 1994, Granato et al. 1997), geometrically thin, optically thick spherical shells (Rowan-Robinson 1995), tapered disks (Efstathiou & Rowan-Robinson 1995, Stenholm 1995), optically thick, flared disks (Manske et al. 1998), clumpy tori (Nenkova et al. 2002), and other more complicated torus geometries (van Bemmel & Dullemond 2003, Schartmann et al. 2005). In order to suppress the 9.7 µm silicate emission feature (which was not detected until very recently by Spitzer; see Section 3.3), some models hypothesized that the dust in AGNs must be large (a < 10 µm) or small silicate grains must be depleted (e.g. see Laor & Draine 1993, Granato & Danese 1994). Some models ascribed the suppression of the 9.7 µm silicate emission feature to clumpiness (Nenkova et al. 2002) or the strong anisotropy of the source radiation (Manske et al. 1998). Apparently, more modeling efforts are required to account for the very recent detection of the 9.7 µm and 18 µm silicate emission features in type 1 AGNs and the recent high resolution IR imaging observations which seem to show that the torus size is no more than a few parsecs (see Elitzur 2006 and references therein). It is well known that the SED modeling alone does not uniquely determine the dust size distribution and the dust spatial distribution.
Acknowledgments. I thank L.C. Ho and J.M. Wang for inviting me to attend this stimulating conference. I also thank B. Cznery, C.M. Gaskell, S.L. Liang, R. Maiolino, and C. Willott for their comments and/or help in preparing for this article. Partial support by NASA/Spitzer theory programs and the University of Missouri Research Board is gratefully acknowledged.