  We report on recent progress in the modelling of the near-IR spectra of young
stellar populations, i.e. populations in which red supergiants (RSGs) are
dominant. First, we discuss the determination of fundamental parameters of RSGs
using fits to their near-IR spectra with new PHOENIX model spectra;
RSG-specific surface abundances are accounted for and effects of the
microturbulence parameter are explored. New population synthesis predictions
are then described and, as an example, it is shown that the spectra of young
star clusters in M82 can be reproduced very well from 0.5 to 2.4 micrometers.
We warn of remaining uncertainties in cluster ages.
