  The Be star phenomenon is related to fast rotation, although the cause of
this fast rotation is not yet clearly established. The basic effects of fast
rotation on the stellar structure are reviewed: oblateness, mixing, anisotropic
winds. The processes governing the evolution of the equatorial velocity of a
single star (transport mechanisms and mass loss) are presented, as well as
their metallicity dependence. The theoretical results are compared to
observations of B and Be stars in the Galaxy and the Magellanic Clouds.
