Photoionization Modelling as a Density Diagnostic of Line Emitting Absorbing Regions in Active Galactic Nuclei Introduction Photoionization simulations of AGN environment The role of gas density Absorption Measure Distribution (AMD) in AGNs Intermediate line region in AGNs Summary and future studies Bibliography This book presents timely work on the nature of the physical processes underpinning two of the basic characteristics of the gas structure in the innermost region of Active Galactic Nuclei (AGN): ionized outflows and emission line regions. In addition it describes physics based methods for estimating the density of the astrophysical plasma surrounding AGN. All numerical computations of the photoionized gas employ the most advanced codes available (CLOUDY and TITAN). Calculations of the radiative transfer are based on the assumption of thermal and ionization equilibrium. Promising preliminary examples of comparison with current observations are included for several individual AGN. All of them suggest that the absorbing emitting gas should have a density on the order of 1012 cm 3. Future observations will provide more objects to verify these results and will allow us to put constraints on the launch radius of ionized outflows and therefore on the mass loading and kinetic energy outflow rates. These rates in turn are crucial to estimating whether the outflows have a significant feedback impact on star formation and metal enrichment in the interstellar medium of the host galaxy. In closing the book discusses a representative example of applying powerful photoionization techniques to explain the complex physics of the AGN environment