  The effect of atomic diffusion on the Main Sequence (MS) of metal-poor low
mass stars is investigated. Since diffusion alters the stellar surface chemical
abundances with respect to their initial values, one must ensure - by
calibrating the initial chemical composition of the theoretical models - that
the surface abundances of the models match the observed ones of the stellar
population under scrutiny. Since the observed surface abundances of subdwarfs
are different from the initial ones due to the effect of diffusion, while the
globular clusters stellar abundances are measured in Red Giants, which have
practically recovered their initial abundances after the dredge-up, the
isochrones to be employed for studying globular clusters and Halo subdwarfs
with the same observational value of [Fe/H] are different and do not coincide.
We find, however,that the current MS-fitting distances derived from HIPPARCOS
subdwarfs using colour corrections from standard isochrones are basically
unaltered when diffusion is taken properly into account; on the other hand, the
absolute ages, the age dispersion, the age-metallicity relation for Halo
subdwarfs, as well as the value of the helium enrichment ratio Delta Y/Delta Z
obtained from the width of the empirical Halo subdwarfs MS, are all
significantly modified when the properly calibrated isochrones with diffusion
are used.
