  The various scenarios proposed for the origin of the multiple,
helium-enriched populations in massive globular clusters are critically
compared to the relevant constraining observations. Among accretion of
helium-rich material by pre-existing stars, star formation out of ejecta from
massive AGB stars or from fast rotating massive stars, and pollution by
Population III stars, only the AGB option appears to be viable. Accretion or
star formation out of outflowing disks would result in a spread of helium
abundances, thus failing to produce the distinct, chemically homogeneous
sub-populations such as those in the clusters $\omega$ Cen and NGC 2808.
Pollution by Population III stars would fail to produce sub-populations
selectively enriched in helium, but maintaining the same abundance of heavy
elements. Still, it is argued that for the AGB option to work two conditions
should be satisfied: i) AGB stars experiencing the hot bottom burning process
(i.e., those more massive than $\sim 3 \msun$) should rapidly eject their
envelope upon arrival on the AGB, thus experiencing just a few third dredge-up
episodes, and ii) clusters with multiple, helium enriched populations should be
the remnants of much more massive systems, such as nucleated dwarf galaxies, as
indeed widely assumed.
