X-ray Diagnostics of Physical Conditions in Warm Absorbers *Yair E. Krongold, N.S. Brickhouse, D.A. Liedahl, F. Nicastro *IA-UNAM Instituto de Astronomía Universidad Nacional Autónoma de México Apartado Postal 70 - 264 Ciudad Universitaria México, D.F., CP 04510 We will discuss diagnostic techniques for photoionized plasmas, focusing on warm absorbers in Active Galactic Nuclei. Despite the great progress boosted by X-ray grating spectrometers in our understanding of the ionized medium in the nuclear environment of active galaxies, little is known about the location, geometry, and origin of these winds. We will review the few cases where these properties can be constrained, and discuss how plasma diagnostic techniques can improve our knowledge in this field, when applied to data obtained with the present and future X-ray instruments.