  The extremes of Active Galactic Nuclei (AGN) variability offer valuable new
insights into the drivers and physics of AGN. We discuss some of the most
extreme cases of AGN variability; the highest amplitudes, deep minima states,
extreme spectral states, Seyfert-type changes, and semi-periodic signals,
including new X-ray observations. The properties of changing-look (CL) AGN are
briefly reviewed and a classification scheme is proposed which encompasses the
variety of CL phenomena; distinguishing slow and fast events, repeat events,
and frozen-look AGN which do not show any emission-line response. Long-term
light curves that are densely covered over multiple years, along with follow-up
spectroscopy, are utilized to gain insight into the underlying variability
mechanisms including accretion disk and broad-line region physics. Remarkable
differences are seen, for instance, in the optical spectral response to extreme
outbursts, implying distinct intrinsic variability mechanisms. Furthermore, we
discuss methods for distinguishing between CL AGN and CL look-alike events
(tidal disruption events or supernovae in dense media). Finally, semi-periodic
light curve variability is addressed and the latest multiwavelength (MWL) light
curve of the binary supermassive black hole (SMBH) candidate OJ 287 from the
MOMO project is presented. Recent results from that project have clearly
established the need for new binary SMBH modelling matching the tight new
constraints from observations, including the measurement of a low (primary)
SMBH mass of ~10^8 Msun which also implies that OJ 287 is no longer in the
regime of near-future pulsar timing arrays.
