Although small in number, massive stars are critical to the formation and
evolution of galaxies. They shape the interstellar medium of galaxies through
their strong winds and ultra-violet radiation, are a major source of the heavy
elements enriching the interstellar medium, and are the progenitors of core-collapse
supernovae and gamma-ray bursts, which are among the most energetic explosions
in the Universe and mark the death of a massive star. Still, our understanding of the
connection between massive stars and supernovae from observations is fairly limited.
In this dissertation, I present new observational evidence that shows the importance
of metallicity, mass-loss, and binarity in the lives and deaths of massive stars.