Globular cluster stars exhibit abundance anomalies which are not shared by their field counterparts. Two global scenarii have been proposed in the past to explain these differences: The primordial enrichment scenario and the evolutionary (or intrinsic) one. Recent observations well below the bump luminosity in globular clusters have raised the weight of the primordial solution. However the stellar sources responsible for these abundance variations have not yet been indubitably identified. In this review we discuss the possible stellar culprits as well as their pros and cons.