Searching for supermassive black hole binaries in the era of multi-messenger astronomy

Supermassive black hole binaries (SMBHBs) are a natural byproduct of galaxy mergers. Yet, they remain undetectable at small separations. A promising method is to identify quasars with periodic variability. I will discuss candidates identified in time-domain surveys, as well as ongoing efforts to confirm their binary nature. I will also describe efforts to detect gravitational waves from SMBHBs with Pulsar Timing Arrays, constraints on tentative binaries in the local universe from the most recent NANOGrav upper limits and the prospects for multi-messenger detections.