M33 possesses a halo but not a traditional bulge. This halo exhibits secondary evidence for intermediate-age populations, such as a halo globular cluster system spanning a wide age range in integrated spectroscopy (Chandar et al. 2002), but the age distribution in the M33 halo has not been constrained via photometry of the low-mass MS stars, as done in M31. Thus, our understanding of the star formation history in the M33 halo is in a state similar to that found in M31 prior to the advent of the HST ACS. For years, M31 was assumed to host an old halo of age > 10 Gyr. The M31 halo has now been probed in multiple locations along the minor axis, spanning the regions where the halo looks more like a bulge and where it looks more like a traditional halo, but in all of these regions it exhibits an extended star formation history (Brown et al. 2006, Brown et al. 2007, Brown et al. 2008). The M33 halo remains the last spiral galaxy halo unexplored via photometry of its low-mass MS stars, leaving its star formation history poorly constrained. Appropriately deep imaging of the M33 halo should be obtained during the remaining HST mission, or it may be many years before such data can be obtained again.
Acknowledgments. I am grateful to my collaborators on the HST / ACS M31 observing programs for their contributions to those projects, and to R. Chandar for useful discussions and suggestions.