  One of the challenges for stellar astrophysics is to reach the point at which
we can undertake reliable spectral synthesis of unresolved populations in
young, star-forming galaxies at high redshift. Here I summarise recent studies
of massive stars in the Galaxy and Magellanic Clouds, which span a range of
metallicities commensurate with those in high-redshift systems, thus providing
an excellent laboratory in which to study the role of environment on stellar
evolution. I also give an overview of observations of luminous supergiants in
external galaxies out to a remarkable 6.7 Mpc, in which we can exploit our
understanding of stellar evolution to study the chemistry and dynamics of the
host systems.
