  We revisited the calculations of the ratios of the Helium-like ion
``triplet'' (resonance, intercombination, and forbidden lines) for Z=6 to 14 (C
V, N VI, O VII, Ne IX, Mg XI, Si XIII) in order to provide temperature, density
and ionization diagnostics for the new high-resolution spectroscopic data of
Chandra and XMM-Newton. Comparing to earlier computations, collisional rates
are updated and the best experimental values for radiative transition
probabilities are used. The influence of an external radiation field
(photo-excitation) and the contribution from unresolved dielectronic satellite
lines to the line ratios are discussed. Collision-dominated plasmas (e.g.
stellar coronae), photo-ionized plasmas (e.g. AGNs) or transient plasmas (e.g.
SNRs) are considered.
