Multiple populations in globular clusters

Recent spectroscopic and photometric observations have revealed that
massive stellar clusters are not made of a single stellar populations
but rather host multiple populations. Also, they should have been
produced within very extensive episodes of star formation, which
involved much more stars than currently present in the globular
clusters.  I will review the evidences for multiple populations in
globular clusters and explore some of its implications on their
formation and early history.  I will also comment on the relation
between the formation of the globular clusters and that of the
galactic halo, and implicitely of all stellar populations
characterized by a high relative frequency of globular clusters.