Well before the discovery of quasars, a special class of galaxies was known to harbor unusually powerful nuclei (Seyfert, 1943). Two primary defining characteristics of these active galactic nuclei (AGNs) are an intense point-like nuclear continuum source and strong, broad emission lines, neither of which could arise from ordinary stellar processes. The very broad emission lines, seen only in permitted transitions, have full-widths at half-maximum of a few thousand to more than ten thousand km s-1. They are produced in the broad-line region (BLR), which is 0.1 pc or less in diameter, and contains many rapidly moving clouds with high electron densities (~ 1010 cm-3). Forbidden lines with typical Doppler velocity widths of 500–1000 km s-1 are emitted from a surrounding ~100 pc narrow-line region (NLR), in which the characteristic electron densities range from 103 to 107 cm-3.