  Blue supergiants of spectral types B and A are the visually brightest stars
in spiral and irregular galaxies, with their most luminous members (at M_V=-10)
outshining entire dwarf galaxies. This characteristic allows us to use them as
probes to study the Local Universe in great detail. In principle, already the
existing large telescopes and instrumentation facilitate quantitative
spectroscopy of these objects as far as the Virgo and Fornax clusters of
galaxies. Beyond their challenging stellar atmospheres and opportunities for
testing sophisticated non-LTE physics they offer numerous applications to
modern astrophysics. Quantitative spectroscopy of supergiants can contribute to
improve our understanding of massive star evolution. Galactic abundance
gradients and abundance patterns, as can be obtained from studies of large
ensembles of supergiants in our own and other galaxies, will foster the
understanding of galactochemical evolution. Finally, they are promising
independent indicators for calibrating the extragalactic distance scale, by
application of the wind momentum-luminosity and the flux-weighted
gravity-luminosity relationships. In view of this large potential, the
objective of this thesis is to improve the status of quantitative spectroscopy
of BA-type supergiants and to provide first applications on a sample of
Galactic and extragalactic targets, within the Local Group and beyond.
