  Massive stars are "cosmic engines" (cf the title of the IAU Symposium 250).
They drive the photometric and chemical evolution of galaxies, inject energy
and momentum through stellar winds and supernova explosions, they modify in
this way the physical state of the interstellar gas and have an impact on star
formation. The evolution of massive stars depends sensitively on the
metallicity which has an impact on the intensity of the line driven stellar
winds and on rotational mixing. We can distinguish four metallicity regimes:
1.- the Pop III regime $0 \le Z < \sim 10^{-10}$; 2.- The low metallicity
regime $10^{-10} \le Z < 0.001$; 3.- The near solar metallicity regime $0.001
\le Z < 0.020$; 4.- The high metallicity regime $0.020 \le Z$. In each of these
metallicity ranges, some specific physical processes occur. In this review we
shall discuss these physical processes and their consequences for
nucleosynthesis and the massive star populations in galaxies. We shall mainly
focus on the effects of axial rotation and mass loss by line driven winds,
although of course other processes like binarity, magnetic fields, transport
processes by internal waves may also play important roles.
