  We present a new set of stellar interior and synthesis models for predicting
the integrated emission from stellar populations in star clusters and galaxies
of arbitrary age and metallicity. This work differs from existing spectral
synthesis codes in a number of important ways, namely (1) the incorporation of
new stellar evolutionary tracks, with sufficient resolution in mass to sample
rapid stages of stellar evolution; (2) a physically consistent treatment of
evolution in the HR diagram, including the approach to the main sequence and
the effects of mass loss on the giant and horizontal-branch phases. Unlike
several existing models, ours yield consistent ages when used to date a coeval
stellar population from a wide range of spectral features and colour indexes.
We rigorously discuss degeneracies in the age-metallicity plane and show that
inclusion of spectral features blueward of 4500 AA, suffices to break any
remaining degeneracy and that with moderate S/N spectra (10 per 20AA,
resolution element) age and metallicity are not degenerate. We also study
sources of systematic errors in deriving the age of a single stellar population
and conclude that they are not larger than 10-15%. We illustrate the use of
single stellar populations by predicting the colors of primordial
proto-galaxies and show that one can first find them and then deduce the form
of the IMF for the early generation of stars in the universe. Finally, we
provide accurate analytic fitting formulas for ultra fast computation of colors
of single stellar populations. The models can be found at
http://www.physics.upenn.edu/~raulj
