There are a variety of galaxies which are classified as nuclearactive, ranging from those which have nuclei which are merely distinguishable from a stellar component all the way to QSO’s. The subject of this lecture will be only those extreme cases (Seyfert I, BL Lac and quasar) which constitute the most intense compact X-ray sources which are presently known. The luminosities of these sources in the 2–10 keV band range upward of 1042 ergs/s (to more than 1046 ergs/s), while those sources classified Seyfert II or NELG are typically less luminous.