X-ray Diagnostics with Full Radiative Transfer (ALI Method) Loïc Chevallier, Anne-Marie Dumont, Olivier Godet, Suzy Collin Observatoire de Paris - section de Meudon 5 place Jules Janssen 92195 MEUDON, FRANCE With the advent of the present and future spatial X-ray missions, it becomes crucial to model correctly the line spectrum of X-ray emitting media. We have built a photoionization code, TITAN, solving the transfer of a thousand lines and of the continuum with the "Accelerated Lambda Iteration" method (ALI), which is most reliable for line transfer. We give some details about this method and a justification of its use as a complement to usual approximations (e.g., escape probability or two-stream approximation) made at present in other codes. Using our TITAN code, new diagnostics based on He-like ion lines of the n=2 complex are presented, in the case of photoionised media, for conditions appropriate for the Warm Absorber in Seyfert 1 galaxies or the X-ray emitting region in Seyfert 2 galaxies. We show in particular the influence of the transfer method used on the G and R ratios of He-like ions.