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We review X-ray plasma diagnostics based on the line ratios of He-like ions.Triplet/singlet line intensities can be used to determine electronictemperature and density, and were first developed for the study of the solarcorona. Since the launches of the X-ray satellites Chandra and XMM-Newton,these diagnostics have been extended and used (from CV to Si XIII) for a widevariety of astrophysical plasmas such as stellar coronae, supernova remnants,solar system objects, active galactic nuclei, and X-ray binaries. Moreover, theintensities of He-like ions can be used to determine the ionization process(es)at work, as well as the distance between the X-ray plasma and the UV emissionsource for example in hot stars. In the near future thanks to the nextgeneration of X-ray satellites (e.g., Astro-H and IXO), higher-Z He-like lines(e.g., iron) will be resolved, allowing plasmas with higher temperatures anddensities to be probed. Moreover, the so-called satellite lines that are formedclosed to parent He-like lines, will provide additional valuable diagnostics todetermine electronic temperature, ionic fraction, departure from ionizationequilibrium and/or from Maxwellian electron distribution.

He-like ions as practical astrophysical plasma diagnostics: From stellar coronae to active galactic nuclei