Searching for supermassive black hole binaries

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 and constraints on tentative binaries in the local universe from the most recent NANOGrav upper limits.