All Galactic black hole binaries with low mass donor stars are transient X-ray sources; we expect most of the X-ray transients observed in external galaxies to be black hole binaries also. Obtaining period estimates for extra-galactic transients is challenging, but the resulting period distribution is an important tool for modeling the evolution history of the host galaxy. We have obtained periods, or upper limits, for 12 transients in M31, using an updated relation between the optical and X-ray luminosities. We have monitored the central region of M31 with Chandra for the last 12 years, and followed up promising transients with HST; 4 sigma B magnitude limits for optical counter parts are 25--29, depending on crowding. We obtain period estimates for each transient for both neutron star and black hole accretors. Periods range from ɟ to 400 hours; these periods are within the range observed in Galactic black hole binaries. More M31 X-ray transients need to be sampled before we can determine whether the M31 and Galactic transient population differ significantly.