  The radio-loud AGN unification model associates powerful radio galaxies with
radio-loud quasars and blazars. In analogy with the radio-quiet scheme, the
nuclear regions of objects showing only narrow emission lines in their optical
spectrum are thought to be obscured to our line-of-sight by a geometrically and
optically thick dusty "torus". In objects showing broad emission lines we
directly observe the innermost parsecs around the central black hole, i.e. the
broad line region and the accretion disk. Radiation from the base of the
relativistic jet dominates the overall emission of blazars, that are seen
almost pole-on. Although the broad picture seems to be well established, there
are several fundamental aspects that are still to be understood. HST studies
have recently shed new light on many issues, from the properties of the nuclei
to the structure of the host galaxies.
