The Microwave Limb Sounder (MLS) on board the Upper Atmosphere Research Satellite (UARS) measured the global distribution of stratospheric ClO over annual cycles for much of the 1990s, albeit with reduced sampling frequency in the latter half of the decade. Here we present an overview of the interannual and interhemispheric variations in the distribution of ClO derived from UARS MLS measurements, with a particular emphasis on enhancements in the winter polar lower stratosphere. Although ClO enhancement within the Arctic vortex is comparable in both magnitude and spatial extent to that in the Antarctic at 465 K (~19 km), a significant interhemispheric disparity is seen at higher altitudes, where maximum ClO abundances, and their spatial extent, are considerably larger in the Antarctic than in the Arctic. The Arctic exhibits much more interannual variability in the magnitude, timing, and distribution of ClO enhancement than does the Antarctic. Nevertheless, during the mid-1990s, when the Arctic lower stratosphere was atypically cold, MLS observed the Arctic vortex to be almost completely filled with enhanced ClO in midwinter to late winter. The peak in the ClO profile is at a higher altitude, and the vertical extent of chlorine activation is larger, in the Antarctic than in the Arctic. The Arctic winter of 1995/1996, however, stands out as having a much more Antarctic-like ClO distribution, with larger maximum ClO abundances, a higher altitude for the profile peak, and greater horizontal and vertical extent of activation than the other winters observed by UARS MLS. In the Southern Hemisphere, ClO becomes enhanced in the sunlit portions of the vortex by at least late May/early June every year, whereas in the Northern Hemisphere, ClO becomes enhanced in mid to late December in some years but not until January in others. Elevated levels of reactive chlorine persist for 4--5 months in the south but only 2--3 months in the north.