  Warm absorbers are an important new probe of the central regions of active
galaxies. So far, they revealed their existence mainly in the soft X-ray
spectral region. In observing and modeling this component, we can learn a lot
about the nature of the warm absorber itself, its relation to other components
of the active nucleus, and the intrinsic AGN X-ray spectral shape.
  Here, we briefly review the basic X-ray spectral features of warm absorbers
(dust-free WAs, dusty WAs, peculiar ~1.1 keV absorption, emission and
reflection components) and then discuss these in more detail based on analyses
of individual objects observed with the X-ray satellite ROSAT. The importance
of XMM in improving our knowledge of the warm material is discussed.

Astrophysics