Massive star clusters are invaluable bridges connecting the interstellar medium, star formation and evolution, the evolution of galaxies, and cosmology. These very complex systems are studied in every wavelength region, from radio to gamma rays, and they are also relevant for gravitational wave detectors. A complete understanding of these systems and of their stellar populations is a challenging task. It requires the exchange of ideas and the collaboration of astrophysicists with observational, theoretical, and numerical expertise in stellar evolution, interstellar matter...