In standard AGN unification schemes, the presence (or absence) of broad emission
lines in AGN optical-UV spectra is dependent upon whether the central engine is obscured by
a dusty torus, which in turn is determined by our viewing angle. In recent years, the discovery of more than a dozen changing-look AGNs whose broad emission lines appear (or disappear) within a span of ~10 years, along with dramatic increases (or decreases) in their bolometric luminosity, challenges our current understanding of AGN unification and black hole accretion.
I will talk about these very interesting objects and what we know about them so far. Focusing on our multi-wavelength studies of Mrk 590, one of the most extreme changing-look AGNs known, I will demonstrate why these objects provide an excellent opportunity for studying supermassive black hole fueling and accretion physics.