  An increasing number of both photometric and spectroscopic observations over
the last years have shown the existence of distinct sub-populations in many
Galactic globular clusters and shattered the paradigm of globulars hosting
single, simple stellar populations.
  These multiple populations manifest themselves in a split of different
evolutionary sequences in the cluster color-magnitude diagrams and in
star-to-star abundance variations. In this paper we will summarize the
observational scenario.
