Some 40 years after the discovery of quasars and 60 years after the publication of C.K. Seyfert's initial observations of high central surface brightness galaxies, we are finally quite certain that active galactic nuclei (AGNs) are powered by accretion onto supermassive collapsed objects. While many important details remain poorly understood, the black-hole/accretion-disk paradigm is now reasonably secure. In contrast, however, we still have no self-consistent models of the nuclear regions that produce (a) the broad emission lines that are so prominent in the UV/optical spectra of AGNs and (b) the strong ab- sorption features seen in the X-ray/UV spectra. Given the proximity of these regions to the central engine, it seems likely that these are some manifestation of the accretion process and related outflow; there is a good deal of empirical evidence that connects these spectral features to disk-related out-flows. Unfortunately, solid information about the broad-line region (BLR) is hard to come by: the BLR is spatially unresolved in even the nearest AGNs and the information in line profiles, sampling only one of six dimensions in phase space, is highly ambiguous.
In this review, we will concentrate on a few things that we can infer about the nature of the BLR, and reverberation mapping, arguably the most promising technique for exploring the nature of the BLR. The scope of this review is limited to a few basic topics and represents, of course, the author's own highly biased personal view.