  We present He-like line ratios (resonance, intercombination and forbidden
lines) for totally and partially photoionized media. For solar plasmas, these
line ratios are already widely used for density and temperature diagnostics of
coronal (collisional) plasmas. In the case of totally and partially
photoionized plasmas, He-like line ratios allow for the determination of the
ionization processes involved in the plasma (photoionization with or without an
additional collisional ionization process), as well as the density and the
electronic temperature. With the new generation of X-ray satellites,
Chandra/AXAF, XMM and Astro-E, it will be feasible to obtain both high spectral
resolution and high sensitivity observations. Thus in the coming years, the
ratios of these three components will be measurable for a large number of
non-solar objects. In particular, these ratios could be applied to the Warm
Absorber-Emitter, commonly present in Active Galactic Nuclei (AGN). A better
understanding of the Warm Absorber connection to other regions (Broad Line
Region, Narrow Line Region) in AGN (Seyferts type-1 and type-2, low- and
high-redshift quasars...) will be an important key to obtaining strong
constraints on unified schemes.
  We have calculated He-like line ratios, for Z=6, 7, 8, 10, 12 and 14, taking
into account the upper level radiative cascades which we have computed for
radiative and dielectronic recombinations and collisional excitation. The
atomic data are tabulated over a wide range of temperatures in order to be used
for interpreting a large variety of astrophysical plasmas.
