  Massive stars play a major role in the evolution of their host galaxies, and
serve as important probes of the distant Universe. It has been established that
the majority of massive stars reside in close binaries and will interact with
their companion stars during their lifetime. Such interactions drastically
alter their life cycles and complicate our understanding of their evolution,
but are also responsible for the production of interesting and exotic
interaction products.
  - Extensive observation campaigns with well-understood detection
sensitivities have allowed to convert the observed properties into intrinsic
characteristics, facilitating a direct comparison to theory.
  - Studies of large samples of massive stars in our Galaxy and the Magellanic
Clouds have unveiled new types of interaction products, providing critical
constraints on the mass transfer phase and the formation of compact objects.
  - The direct detection of gravitational waves has revolutionized the study of
stellar mass compact objects, providing a new window to study massive star
evolution. Their formation processes are, however, still unclear. The known
sample of compact object mergers will grow by orders of magnitude in the coming
decade, turning into the best understood astrophysical population.
