The Event Horizon Telescope has released total intensity images of the Messier 87* and Sagittarius A* accretion flows; polarized images have been released for M 87*, and are promised for Sgr A*. These images constitute a rich vein for theoretical constraints on the black hole accretion flow system, but a trustworthy measurement of either spin remains elusive. Spin nonetheless remains a high priority, as the black hole angular momentum is deeply linked to its galactic co-evolutionary history. In my talk, I will discuss the current theoretical traction on supermassive black hole spin and the state of spin measurements using existing and future VLBI data, including measurements of the black hole photon ring, inference of near-horizon magnetic field structure, and next-generation spacetime/emissivity inference codes.
