A Unified View of How the Study of Emission Lines Furthers our Knowledge of AGN

A global view is given of how emission lines have been, and may in the future, be used to enhance our understanding of AGN. Lines from the microwave to the X-ray bands all contribute. Although we have a deep understanding of the physical processes by which line photons are generated, when the circumstances of line emission are complicated, models become too unreliable to provide strong inferences about the rest of the AGN system. At present, the lines which appear the most promising for helping answer the most important questions about AGN are the 22~GHz H_2 O rotational transition and the 6.4 keV Fe Kalpha fluorescence.