  Photospheric radiation momentum is efficiently transferred by absorption
through metal lines to the gaseous matter in the atmospheres of massive stars,
sustaining strong winds and mass loss rates. Not only is this critical for the
evolution of such stars, it also provides us with diagnostic UV/optical
spectral lines for the determination of mass loss rates, chemical abundances
and absolute stellar luminosities. We review the mechanisms which render the
wind parameters sensitive to the chemical composition, through the statistics
of the wind-driving lines. Observational evidence in support of the radiation
driven wind theory is presented, with special regard to the Wind
Momentum-Luminosity Relationship and the blue supergiant work carried out by
our team in galaxies outside of the Local Group.
