I will review some of the recent work on the RR Lyrae populations in M31 and M33. The capabilities of the Hubble Space Telescope and 10-meter class ground-based telescopes have made it possible to reliably identify and characterize RR Lyrae variables in these two galaxies. This is important because RR Lyraes are the 'Swiss Army knives' of astronomy in the sense that they have multiple and varied uses for probing the formation and evolution of galaxies. I will describe the diversity of ways that RR Lyraes are useful in this regard and what they reveal about the properties of M31 and M33.