The idea that Globular Clusters stars are a classic example of a single stellar
population, made up of objects born at the same time, and sharing the
same chemical composition, has been challenged in the last decades by
many spectroscopic surveys of Globular Cluster, that evidentiated
clear star-to-star differences in their surface chemistry, that assume the
form of well defined chemical patterns, such as the Na-O and Mg-Al
anticorrelations. In this contribution we discuss the self-enrichment scenario,
i.e. the idea that at least two stellar generations may exist in Globular Clusters,
the second of which is formed from the gas ejected during the AGB phase by 
the intermediate mass stars belonging to the first generation.
Recent photometric evidences supporting this hypothesis are also commented.