1 INTRODUCTION

2 BINDING OBSERVATIONAL CONSTRAINTS

3 CHECKING GC FORMATION SCENARIOS
3.1 Supermassive Stars
3.2 Fast Rotating Massive Stars
3.3 Massive Interacting Binaries
3.4 AGB Stars
3.5 Multiple Stellar Populations or Multiple Stellar Generations?
3.6 Summary of the Cross Check of Scenarios and Constraints

4 SAVING THE AGB OPTION?
4.1 The Mass Budget and the IMF of 2G Stars
4.2 Sodium-Oxygen Anti-correlation, and the Like
4.3 Oxygen Depletion without Much Helium Enrichment

5 SUMMARY AND CONCLUSIONS