  Although soft X-ray absorption features in AGN were discovered almost ten
years ago, the nature and location of the gas creating them has remained
controversial. However, by making use of the newly-available high-resolution
spectra provided by XMM-Newton and Chandra, we should be able to make
substantial advances. The first such spectra indicate that multiple ionization
states coexist in the absorber; this is a natural consequence of
photoionization physics. Photoionized evaporation in the presence of a copious
mass source locks the ratio of ionizing intensity to pressure to a critical
value. A broad range of temperatures can all coexist in equilibrium for this
value of the ratio of ionizing intensity to pressure. Consequently, the flow is
expected to be strongly inhomogeneous in temperature. The inferred distance of
this material from the source of ionizing radiation depends on how much matter
exists at the highest-obtainable temperature. This distance can be measured by
monitoring how ionic column densities respond to changes in the ionizing
continuum on timescales of days to years.
