Ionized gas in active galactic nuclei

This review gives an introduction to the physics of ionized gas in active galactic nuclei (AGN). The main topics are: Definition of the AGN phenomenon, methods for discovering AGN, basic AGN observations, photoionization and recombination of low density plasma, energy balance and gas temperature, the spectrum of ionized gas, dust and reddening, the motion of ionized gas under the influence of strong radiation fields, the broad line region (BLR), the narrow line region (NLR), and the highly ionized outflowing gas in AGN.