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.

2 pages. Dissertation Summary, accepted for publication in PASP. Thesis may be retrieved from http://edoc.ub.uni-muenchen.de/archive/00000082/01/Przybilla_Norbert.pdf