An increasing number of observations have confirmed the presence of multiple stellar populations in Galactic globular clusters. Multiple populations evidence come from the complex chemical pattern of stars hosted in GCs and from the split or broadening of different evolutionary sequences in the color-magnitude diagrams. Multiple stellar populations have been identified in Galactic and Magellanic Cloud clusters, as well as in external galaxies. In this paper I will summarize the observational facts.

ABSTRACT
1. INTRODUCTION
2. PHOTOMETRIC EVIDENCE
    2.1. Main Sequence Multiplicity
    2.2. Sub Giant Branch Multiplicity
    2.3. Red Giant Branch Split
    2.4. Multiple Populations in Other GC Systems
3. SPECTROSCOPIC EVIDENCE
4. SUMMARY AND CONCLUSIONS
REFERENCES

• GIAMPAOLO PIOTTO(Dipartimento di Astronomia, Universit'a di Padova)