  The efficient use of near-IR data in studies of external stellar populations
depends on our ability to recognize the nature of the predominant sources of
light, and to interprete these findings in terms of age and metallicity. Here
we focus on elderly post-starburst populations, with ages of 0.1-1 Gyr. New
models confirm that they are indeed expected to display specific spectral
signatures in the near-IR, due to variable M stars of the asymptotic giant
branch and to carbon stars. The signatures depend on age and metallicity. We
summarize the status of current quantitative predictions and emphasize the
importance of an empirical calibration of the spectral synthesis models.
