Massive star evolution along the cosmic history: some puzzles and perspectives
Georges Meynet

In the early Universe massive stars played a key role in the early chemical evolution of galaxies and in injecting important amount of ionising radiation in their environments. Their high luminosities, as well as their explosion in supernovae made them objects which can be detected even in far distant galaxies, allowing to use them as tools for inferring the history of the star formation in galaxies. When populations of massive stars can be observed, the relative fraction of Wolf-Rayet to O-type stars, of Luminous Blue Variables to O-type stars, of blue to red supergiants can teach a lot about the physics of these objects as well as on the star formation history. The massive stars in the solar neighborhood represents the benchmark for checking the models and study physical effects which require proximity for being observed and detected. In this talk, after recalling the important physical processes affecting the evolution of massive stars, I shall discuss, using recent observational and theoretical results, a few key questions which represent interesting challenges in the frame of massive star researches.