  HST has recently revealed that many among the most massive globular clusters
harbor multiple stellar populations, and --most surprisingly-- some of them are
extremely helium rich. How these clusters managed to generate such complex
stellar populations, and what processes let to so dramatic helium enrichment,
is today one of the most exciting puzzles in the astrophysics of stellar
systems. HST has also been instrumental in demonstrating that both the bulge of
our own Galaxy and that of M31 are dominated by old stellar populations, coeval
to galactic globular clusters. Ultradeep HST imaging has also demonstrated that
a major component of the M31 halo is metal rich and much younger than old
globular clusters.
