  Major flows of ionized gas are thought to be present in the nuclei of most
luminous QSOs, and absorption by these flows often has a significant effect
upon the observed X-ray continuum from the black hole region. We briefly
discuss X-ray studies of this gas and attempts to determine its geometry,
dynamics, and ionization physics. Our focus is on X-ray warm absorber QSOs,
Broad Absorption Line QSOs, and soft X-ray weak QSOs. We also discuss some
prospects for further study with the next generation of X-ray observatories.

Astrophysics