  This White Paper explores advances in the study of Active Galaxies which will
be enabled by new observing capabilities at MeV energies (hard X-rays to
gamma-rays; 0.1-1000 MeV), with a focus on multi-wavelength synergies. This
spectral window, covering four decades in energy, is one of the last frontiers
for which we lack sensitive observations. Only the COMPTEL mission, which flew
in the 1990s, has significantly probed this energy range, detecting a handful
of AGN. In comparison, the currently active Fermi Gamma-ray Space Telescope,
observing at the adjacent range of 0.1-100 GeV, is 100-1000 times more
sensitive. This White Paper describes advances to be made in the study of
sources as diverse as tidal disruption events, jetted AGN of all classes
(blazars, compact steep-spectrum sources, radio galaxies and relics) as well as
radio-quiet AGN, most of which would be detected for the first time in this
energy regime. New and existing technologies will enable MeV observations at
least 50-100 times more sensitive than COMPTEL, revealing new source
populations and addressing several open questions, including the nature of the
corona emission in non-jetted AGN, the precise level of the optical
extragalactic background light, the accretion mode in low-luminosity AGN, and
the structure and particle content of extragalactic jets.
