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Article Contents

ABSTRACT

1.INTRODUCTION

2.GAS DIAGNOSTICS

2.1.Extinction (A_V)

2.2.Electron density (n_e)

2.3.Electron Temperature (T_e)

3.IONIZATION AND EXCITATION MECHANISMS

3.1.Photoionization: Characterizing the UV radiation field

3.2.Photodissocation regions

3.3.Shocks

4.ABUNDANCES

4.1.Optically "invisible" gas

4.2.Ionic abundance ratios

4.3.Altered & evolving abundances

5.SOME CURRENT ISSUES: A SAMPLER

5.1.The origin of strong [Fe II] lines

5.2.Curiosities at the Galactic Center

5.3.Infrared spectroscopy of AGN

6.CONCLUDING REMARKS

REFERENCES