M31 has been monitored by the HRC and ACIS detectors aboard Chandra from 1999 to 2002. We report here a summary of properties of X-ray sources in M31 with a focus on an ongoing Chandra/HST program to monitor X-ray transients in M31. By comparing individual X-ray images, more than 100 sources were found to be variable (half the total number observed) on a time scale of months. Combining our sample with all of the available transients found in the literature, there are ~30 transients in M31 and M32 detected by Chandra and XMM. We also perform follow-up observations with HST to search for the UV counterparts of transient sources. While no definitive UV counterparts have been found so far, the X-ray brightest of these transients has the strongest candidate counterpart. The discovery of repeated outbursts from three of the transients yields new direct information on duty cycles of transients in M31.