Abstract
Since Baldwin, Phillips, & Terlevich and Osterbrock & Veilleux popularized their use, optical diagnostic plots have been used to attempt to distinguish between the various modes of excitation in nuclear emission regions of both active and normal galaxies. Recently, great progress has been made in gathering new observational data, and in building theoretical models to describe the results. We will review these results, and attempt to show what parameters of the nuclear emission regions can be unequivocally derived from optical and UV diagnostics, and point the way towards making further progress in this problem.
