Two Supermassive Black Holes are Better than One

Supermassive black holes, once thought to be theoretical novelties, are now considered to play a major role in many astrophysical phenomena including galaxy evolution. Now that we live in the era of gravitational wave observations, it is interesting to look forward to a time when we can detect gravitational waves from supermassive black hole coalescence. A major question remains: Do supermassive black holes merge?  I will review the case for supermassive black holes role in the universe, focusing on the black hole mass scaling relations. Then I will introduce a new, empirical scaling relation that can be used for black hole mass estimation. Finally I will discuss the prospects and pitfalls of searching for dual and binary AGNs along with recent devlopments.  These include (1) closer inspection of time-domain-identified binary candidates; (2) a Bayesian framework for determining duality in a Chandra observation; and (3) spectroscopic and time-domain identification of low-mass-ratio binary AGN.