Massive star clusters contain a substantial population of stars with unusual abundances of light elements, with chemical patterns indicative of hot hydrogen burning. The source of this material, the large number of stars with this signature, and its presence only in massive clusters all remain unsolved puzzles of this 'multiple population' problem. In this discussion, I will argue that a complete solution must involve a combined understanding of the stellar populations found in massive star clusters and how those stellar populations are modified, and can themselves modify, the complex star cluster assembly process.