Active galactic nuclei constitute a somewhat vaguely defined class of objects. It is very well possible that most galaxies have nuclei and that these are active in the sense that there is an energy source in addition to the thermonuclear sources inside the constituent stars. Something is called an AGN if this activity is “substantial” in some characteristic, but the quantitative meaning of this is unclear and tied up with experimental or observational possibilities. For example, in the nucleus of the “normal” galaxy M 81 very faint broad wings (± 3000 km/sec) have been detected in Hα and Hβ (Peimbert and Torres-Peimbert 1981, Shuder and Osterbrock 1981, Filippenko and Sargent 1988). Such a detection is important because it shows that high velocity material is present, which either has been accelerated by some process or which is moving in a very deep potential well. It cannot be excluded that some such high velocity gas is present in all “normal” nuclei, but that in most cases the quantity is still smaller than in M 81. This example shows that it is not enough to define certain phenomena as corresponding to AGN, but that it is necessary to quantify the activity by measurable criteria. In addition, we have to define the meaning of “nuclear”. Is it a region 1 pc or 1 kpc across ? For the moment, we shall take a rather generous definition and not inquire too closely about its precise meaning.