The central region of M31 has been monitored with Chandra > 120 times over the last 11 years. In this region we find X-ray sources corresponding to 35 out of 420 globular clusters; these are highly likely to be X-ray binaries. We have created long-term, calibrated lightcurves for all 35 sources, and will present highlights of our variability survey. We have detected significant variability in all the sources with 0.3-10 keV luminosity > 2x10E+36 erg/s. Since the emission spectra of background active galaxies often resemble those of X-ray binaries, the long term variability will be a valuable tool for identifying X-ray binaries in the remaining 400 sources in our field. This work is funded by Chandra grant GO9-0100X and HST grant GO-1101.