So far I have discussed X-ray absorption in AGNs which were discovered and classified in the optical. However, an increasing number of obscured powerful AGNs has been discovered by means of hard X-ray observations in galaxies which are optically classified as starburst or LINER. Probably, in these objects the obscuring medium hides also the NLR. Alternatively, the nuclear ionizing source might be completely embedded and obscured in all directions. The most spectacular case is the nearby (4 Mpc) edge-on galaxy NGC4945, which hosts one of the brightest AGNs at 100 keV (Done et al. 1996) The powerful X-ray nucleus is obscured by a column of ~ 5 x 1024 cm-2 along our line of sight. However, it seems that the nucleus is heavily obscured in all directions. Indeed, optical to mid-IR observations were unable to detect any indication of the AGN activity: at these wavelengths the central region is characterized by a starburst with a spectacular superwind cavity where LINER-like lines are produced (Maiolino et al. 2000a, Marconi et al. 2000). A case quite similar to NGC4945, but at much higher luminosities, is NGC6240. The optical spectrum of this strongly interacting system shows only weak LINER-like emission lines and the mid-IR properties are similar to starburst galaxies (Lutz et al. 1999). However, hard X-ray observations have detected the presence of a heavily obscured AGN whose intrinsic luminosity is in the QSO range (Vignati et al. 1999). Other luminous IR galaxies, optically classified as starburst or LINER, might also host completely hidden AGNs (Risaliti et al. 2000).
Some of the luminous hard X-ray sources recently discovered by deep Chandra surveys have optical and near-IR counterparts that show little or no evidence for AGN activity (eg. Mushotzky et al. 2000, Fiore et al. 2000). Possibly, many of these objects are the analogous at higher redshift of the local, completely hidden AGNs discussed above.