Most of the AGN emission in the universe is obscured. Locally, optically obscured ("type II") AGN outnumber unobscured AGN ("type I") by a factor of ~ 4 (Maiolino & Rieke 1995). This factor could be even higher at redshifts z = 0.5 - 1, according to synthesis models of the X-ray background (Gilli et al. 2001). Absorption introduces a further spread in the emission properties of AGN, since the observed SEDs depend both on the intrinsic emission (which has a large dispersion) and on the amount, composition, and geometry of the absorber. In general, the main absorption mechanisms in AGN are the following:
(1) Line absorption due to atomic transitions. The strongest features observed in AGN spectra ("Narrow Absorption Lines" and "Broad Absorption Lines" in the UV, "Warm Absorption" in the soft X-rays) are mainly due to resonant absorption lines.
(2) Continuum absorption in the IR to UV due to dust.
(3) Continuum absorption (or scattering) in the X-rays due to photoelectric absorption by dust and gas.
The physical state (temperature, density), column density, and metallicity of the gas, the chemical composition of the dust, the dust-to-gas ratio, and the composition of the dust grains are all elements that affect the observed SED. Here we review the main emission properties of obscured AGN, focusing mostly on the actual observational results and only briefly discussing their physical interpretation.