  We review some important observed properties of massive stars. Then we
discuss how mass loss and rotation affect their evolution and help in giving
better fits to observational constraints. Consequences for nucleosynthesis at
different metallicities are discussed. Mass loss appear to be the key feature
at high metallicity, while rotation is likely dominant at low and very low
metallicities. We discuss various indications supporting the view that very
metal poor stars had their evolution strongly affected by rotational mixing.
Many features, like the origin of primary nitrogen at low metallicity, that of
the C-rich extremely metal poor halo stars, of He-rich stars in massive
globular clusters, of the O-Na anticorrelation in globular clusters may be
related to the existence of a population of very fast rotating metal poor stars
that we tentatively call the {\it spinstars}. A fraction of these {\it
spinstars} may also be the progenitors of GRB in very metal poor regions. They
may avoid pair instability explosion due to the heavy mass loss undergone
during their early evolutionary phases and be, dependent on their frequency,
important sources of ionising photons in the early Universe.
