High-resolution X-ray spectroscopy of Active Galactic Nuclei has become
possible thanks to the launch of XMM-Newton and Chandra with their grating
spectrometers, and will be explored further after the expected launch of
ASTRO-E2 with its XRS detector. In several AGN the X-ray spectra show the
signatures of on outflowing, photoionized wind. Also several X-ray binaries
show a similar imprint of a photoionized gas. The clearest signatures are
formed by the broad range of absorption lines, mostly from the ground states of
a wide range of ionization states of the abundant elements. In addition to
absorption lines due to the valence electrons, the spectra show many
inner-shell absorption lines. Examples are the K-shell transitions of the most
abundant metal, oxygen, in the 19-23 Angstrom band, and 2p-3d transitions of
lowly ionized iron in the 15-17 Angstrom region. These transitions have an
extremely important diagnostic value, as other transitions of the same ions
frequently occur in the unobservable extreme ultraviolet. Several of these
inner-shell transitions, however, lack accurate experimental or theoretical
wavelengths, which makes the spectral analysis complicated and ambiguous. This
is even more the case for transitions from metastable levels, which can be used
as density diagnostics. Finally, attention is payed to the role of atomic data
in the photoionization equilibrium calculations. Uncertainties in for example
dielectronic recombination rates cause large uncertainties in the predicted
absorption line strengths.