Published in "Physics of Active Galactic Nuclei at all Scales", eds. Danielle Alloin, Rachel Johnson and Paulina Lira. Lecture Notes in Physics, Vol. 693, p.77, 2006.

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Abstract: We review the basic observed and inferred properties of the broad emission-line region in AGNs, as well as the basics of the reverberation-mapping technique that can be used to determine the size and structure of the region. We argue that the current best evidence points to a multi-component line-emitting region, with a disk-like structure, possibly an extension of the accretion disk itself, and a disk wind being strong candidates for the origin of the broad-line emission.

Table of Contents

• INTRODUCTION
• BASIC INFERENCES ABOUT THE LINE-EMITTING GAS
• BROAD EMISSION-LINE VARIABILITY AND THEORY OF REVERBERATION MAPPING
• Basic Assumptions
• Isodelay Surfaces
• Velocity-Delay Maps
• REVERBERATION RESULTS
• Size of the BLR and Scaling with Luminosity
• Reverberation and AGN Black Hole Masses
• WHAT IS THE BLR?
• "Cloud" Models
• "Bloated Stars" Model
• Double-Peaked Emission Lines
• Disk Winds
• WHAT WILL IT TAKE TO MAP THE BROAD-LINE REGION?
• SOME OBVIOUS QUESTIONS TO KEEP IN MIND
• SUMMARY
• REFERENCES