2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/98795Warm 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.6 pages, 2 figures (needs aaspp4.sty); to appear in the proceedings of "The First XMM Workshop" (Noordwijk, Sept 30 - Oct 2, 1998)Astrophysicstext