  We highlight some of the principal results from our recent Hubble Space
Telescope studies of quasars and radio galaxies. The hosts of these powerful
AGN are normal massive ellipticals which lie on the region of the fundamental
plane populated predominantly by massive ellipticals with boxy isophotes and
distinct cores. The hosts of the radio-loud sources are on average 1.5 times
brighter than their radio-quiet counterparts and appear to lie above a mass
threshold of 4 x 10^11 solar masses. This suggests that black holes more
massive than 5 x 10^8 solar masses are required to produce a powerful radio
source. However we show that this apparent threshold appears to be a
consequence of an upper bound on radio output which is a strong function of
black-hole mass, L_5GHz proportional to M_bh^2.5. This steep mass dependence
can explain why the hosts of the most powerful radio sources are good standard
candles. Such objects were certainly fully assembled by z = 1, and appear to
have formed the bulk of their stars prior to z = 3.
