  A brief review on globular cluster sub-populations in galaxies, and their
constraints on galaxy formation and evolution is given. The metal-poor and
metal-rich sub-populations are put in a historical context, and their
properties, as known to date, are summarized. We review why the study of these
sub-populations is extremely useful for the study of galaxy formation and
evolution, but highlight a few caveats with the current interpretations. We
re-visit the current globular cluster system formation scenarios and show how
they boil down to a single scenario for the metal-poor clusters (namely the
formation in ``universal'', small fragments at high $z$) and that a
hierarchical formation seems favored for the metal-rich clusters.
