Shortly after the discovery of quasars, it was understood that these objects are truely remarkable, not only in their physical nature but also in their cosmological evolution. Schmidt (1968, 1970) observed a very sharp decline in the space density of quasars, over roughly a factor of 100 from z ~ 2 to z ~ 0. This decline was so remarkable, that it could be credibly demonstrated from a sample as small as 20 objects. The interpretation was supported by V / Vmax tests, developed for this purpose at the time. The strong evolution clearly applied to both, objects selected optically and those selected from their radio emission. At the time it suggested that quasars were the most dramatically evolving population of objects in the Universe.
It was later understood that quasars are only one part of a larger
population of objects commonly called Active Galactic Nuclei or
AGN. From an optical perspective, a defining characteristic of AGN is
non-stellar continuum radiation. More sensitive observations showed
that also light from luminous and young stars is sometimes found in
AGN, suggesting that star formation may be occuring in the host galaxy
and may have a common trigger with the nuclear activity. AGN have been
observed over a wide range of luminosities which presently covers eight
orders of magnitude, across
MB 
[- 10, - 30]. This range poses a challenge for observations aiming at
understanding the phenomenon: at the lowest luminosities the host galaxy
outshines the active nucleus, making it hard to detect. At the highest
luminosities, the AGN outshines the host galaxy, making the latter
almost impossible to detect, let alone to investigate. Traditionally,
unobscured AGN at MB < - 23 have been called
QSOs. This definition does not reflect any physical limits, but is
motivated by observational practicalities: here the AGN is luminous
enough to dominate the combined spectrum over the host galaxy, making
its AGN nature easy to recognize.
This article focusses on the more luminous half, MB < - 20, of the range covered by AGN, where > 95% of the total luminosity density is emitted. Besides these bright unobscured type-1 objects, there is an entire class of optically obscured type-2 AGN with a very significant contribution to the cosmic black hole accretion history. However, being optically obscured makes these objects hard to find and study from an optical point of view, although their host galaxies are clearly observable. Hence, this article will give more weight to type-1 objects, here called QSOs irrespective of their luminosity.