  The observed abundance variations in globular cluster red giants indicate
that these stars may be mixing helium from the hydrogen shell outward into the
envelope, presumably as a result of internal rotation. We have investigated the
implications of such helium mixing for both the red- giant-branch (RGB) and
horizontal-branch (HB) phases by computing a number of noncanonical
evolutionary sequences for different assumed mixing depths and mass loss rates.
We find that the helium-mixed models evolve to higher luminosities during the
RGB phase and consequently lose more mass than their canonical counterparts.
This enhanced mass loss together with the higher envelope helium abundances of
the helium-mixed models produces a markedly bluer and somewhat brighter HB
morphology. As a result, helium mixing can mimic age as a 2nd parameter and can
reduce the ages of the metal-poor globular clusters derived from the luminosity
difference between the HB and the main-sequence turnoff. Helium mixing might
also lead to a larger RR Lyrae period shift and to a steeper slope for the RR
Lyrae luminosity - metallicity relation if the mixing is more extensive at low
metallicities, as suggested by the observed abundance variations. We discuss
the implications of helium mixing for a number of other topics including the
low gravities of the blue HB stars, the origin of the extreme HB stars, and the
evolutionary status of the sdO stars. A variety of observational tests are
presented to test this helium-mixing scenario.
