  Massive stars can develop into tepid supergiants at several stages of their
post main-sequence evolution, prior to core He-burning, on a blue loop, or
close to the final supernova explosion. We discuss observational constraints on
models of massive star evolution obtained from the analysis of a sample of
Galactic supergiants and put them in the context of the cosmic abundance
standard as recently proposed from the study of their OB-type progenitors
(Z=0.014 for stars in the solar neighbourhood). High-precision abundance
analyses for He and CNO, with uncertainties as low as ~10-20%, trace the
transport efficiency of nuclear-processed material to the stellar surface,
either by rotational mixing or during the first dredge-up. A mixing efficiency
higher by a factor ~2 than predicted by current evolution models for rotating
stars is indicated, implying that additional effects need to be considered in
evolutionary models like e.g. the interplay of circulation and magnetic fields.
Blue loops are suggested to extend to higher masses and to higher Teff than
predicted by the current generation of stellar evolution models.
