  Rotation can have severe consequences for the evolution of massive stars. It
is now considered as one of the main parameters, alongside mass and metallicity
that determine the final fate of single stars. In massive, fast rotating stars
mixing processes induced by rotation may be so efficient that helium produced
in the center is mixed throughout the envelope. Such stars evolve almost
chemically homogeneously. At low metallicity they remain blue and compact,
while they gradually evolve into Wolf-Rayet stars and possibly into progenitors
of long gamma-ray bursts.
  In binaries this type of evolution may occur because of (I) tides in very
close binaries, as a result of (II) spin up by mass transfer, as result of
(III) a merger of the two stars and (IV) when one of the components in the
binary was born with a very high initial rotation rate. As these stars stay
compact, the evolutionary channels are very different from what classical
binary evolutionary models predict. In this contribution we discuss examples of
nearly chemically homogeneous evolution in very close tidally-locked binaries.
Even in such very close massive binaries, the stars may remain compact and
avoid mass transfer, while Roche lobe overflow and a merger would be inevitable
in the classical picture. This type of evolution may provide an alternative
path to form tight Wolf-Rayet binaries and massive black hole binaries.
