  Observations of star-forming galaxies in the distant Universe have confirmed
the importance of massive stars in shaping galaxy emission and evolution.
Distant stellar populations are unresolved, and the limited data available must
be interpreted in the context of stellar population models. Understanding these
populations, and their evolution with age and heavy element content is key to
interpreting processes such as supernovae, cosmic reionization and the chemical
enrichment of the Universe. With the upcoming launch of JWST and observations
of galaxies within a billion years of the Big Bang, the uncertainties in
modelling massive stars - particularly their interactions with binary
companions - are becoming increasingly important to our interpretation of the
high redshift Universe. In turn, observations of distant stellar populations
provide ever stronger tests against which to gauge the success of, and flaws
in, current massive star models. Here we briefly review the current status
binary stellar population synthesis.
