Although emission lines in the nuclei of galaxies were recognized at the beginning of the twentieth century, a half century more would pass before active galactic nuclei (AGN) became a focus of intense research effort. The leisurely pace of optical discoveries in the first half of the century gave way to the fierce competition of radio work in the 1950s. The race has never let up. Today, AGN are a focus of observational effort in every frequency band from radio to gamma rays. Several of these bands involve emission lines as well as continuum. AGN theory centers on extreme gravity and black holes, among the most exotic concepts of modern astrophysics. Ultrarelativistic particles, magnetic fields, hydrodynamics, and radiative transfer all come into play. In addition, AGN relate to the question of galactic evolution in general. For most of the time since the recognition of quasar redshifts in 1963, these objects have reigned as the most luminous and distant objects in the Universe. Their use as probes of intervening matter on cosmic scales adds a further dimension to the importance of AGN.
For all these reasons, the enormous effort to describe and explain AGN in all their variety and complexity is quite natural. We are far from having a detailed and certain understanding of AGN. However, the working hypothesis that they involve at their core a supermassive black hole producing energy by accretion of gas has little serious competition today. If this picture is confirmed, then the past decade may be seen as a time when AGN research shifted from guessing the nature of AGN to trying to prove it.
Although the story is not finished, this seems a good time to take stock of the progress that has been made. The present short summary is intended to give students of AGN an account of some of the key developments in AGN research. The goal is to bring the story to the point where a contemporary review of some aspect of AGN might begin its detailed discussion. Thus, various threads typically are followed to a significant point in the 1980s.
I have attempted to trace the important developments without excessive technical detail, relying on published sources, my own recollections, and conversations with a number of researchers. The focus is on the actual active nucleus. Fascinating aspects such as intervening absorption lines, statistical surveys, and links to galactic evolution receive relatively little discussion. The volume of literature is such that only a tiny fraction of the important papers can be cited.