The He-Like Triplet Ratios as Powerful Plasma Diagnostic Tools Delphine Porquet MPI-Extraterrestrische Physik P.O. Box 1312 Garching bei Munchen, 85741 Germany With the advent of the new generation of X-ray satellites (Chandra and XMM-Newton), X-ray spectroscopy with unprecedented spectral resolution for extra-solar objects is now possible for the first time. In particular, the He-like line triplet ratios are powerful diagnostic tools for a variety of plasmas such as collisional (e.g., stellar coronae), and/or photo-ionized (e.g., Active Galactic Nuclei) plasmas. I will show how the triplet line ratios can be used to infer the density, the electron temperature (and ionization processes) of the plasma. I will review the advantages and limitations of these diagnostics and as well the perspectives with Astro-E2 and Constellation X.