  We review some of the main physical and statistical properties of the X-ray
absorber in AGNs. In particular, we review the distribution of the absorbing
column density inferred from X-ray observations of various AGN samples. We
discuss the location of the X-ray absorber and the relation with the dust
absorption at optical and infrared wavelengths. Finally, we shortly review the
recent findings on X-ray absorption at high luminosities and at high redshift.
