  Observations of star-forming galaxies in the distant Universe (z > 2) are
starting to confirm the importance of massive stars in shaping galaxy emission
and evolution. Inevitably, these distant stellar populations are unresolved,
and the limited data available must be interpreted in the context of stellar
population synthesis models. With the imminent launch of JWST and the prospect
of spectral observations of galaxies within a gigayear of the Big Bang, the
uncertainties in modelling of massive stars are becoming increasingly important
to our interpretation of the high redshift Universe. In turn, these
observations of distant stellar populations will provide ever stronger tests
against which to gauge the success of, and flaws in, current massive star
models.
