Massive stars are essentially the driving force of the entire universe – in terms of energy, momentum, and baryonic cycling through galaxies – over cosmic time. Without them, the present universe would simply not exist.
Despite their importance and the remarkable progress made by the Astrophysical community in understanding their physical properties and the interconnected processes that drive their end-to-end life cycle, many critical questions remain open. This is a consequence of their complex formation process and subsequent evolution. In brief, the whole end-to-end life-cycle of these extreme stellar objects is known to be dominated by (1) their initial mass, which is intimately linked to the star formation process, (2) the large amount of angular momentum remaining in the star once it has reached the Zero Age Main Sequence, and how this is shared between their cores and surfaces along their evolution, (3) mass transfer processes occurring in the high percentage of stars of this type born in binary and multiple systems, and (4) the strong -- sometimes steady, sometimes eruptive – mass loss events driven by the interaction between the stellar radiation and the outer layers of these stars. Overall, this leads to a still not fully understood diversity of possible final fates (represented by a variety of core-collapse supernovae events) and end products (including isolated neutron stars and black holes, and binary systems comprising two of these stellar corpses, which can eventually lead to a gravitational wave event if they merge).