I’ll discuss an old technique called intensity interferometry, pioneered by Hanbury Brown and Twiss (HBT) that allows us to achieve sub-microarcsecond angular resolution using ordinary ground-based optical telescopes observing visible light.  This technique is best suited for compact, bright sources, and I will discuss applications of intensity interferometry observations of bright, supermassive black holes called active galactic nuclei (AGN).  I’ll discuss three possible applications, including measurement of the Hubble constant H0, probing the innermost structure of quasar accretion disks, and testing GR through observations of photon rings near the black hole horizon.