  We have incorporated the latest release of the Padova models into the
evolutionary synthesis code Starburst99. The Padova tracks were extended to
include the full asymptotic giant branch (AGB) evolution until the final
thermal pulse over the mass range 0.9 to 5 solar mass. With this addition,
Starburst99 accounts for all stellar phases that contribute to the integrated
light of a stellar population with arbitrary age from the extreme ultraviolet
to the near-infrared. AGB stars are important for ages between 0.1 and 2 Gyr,
with their contribution increasing at longer wavelengths. We investigate
similarities and differences between the model predictions by the Geneva and
the Padova tracks. The differences are particularly pronounced at ages > 1 Gyr,
when incompleteness sets in for the Geneva models. We also perform detailed
comparisons with the predictions of other major synthesis codes and found
excellent agreement. Our synthesized optical colors are compared to
observations of old, intermediate-age, and young populations. Excellent
agreement is found for the old globular cluster system of NGC 5128 and for old
and intermediate-age clusters in NGC 4038/39. In contrast, the models fail for
red supergiant dominated populations with sub-solar abundances. This failure
can be traced back to incorrect red supergiant parameters in the stellar
evolutionary tracks. Our models and the synthesis code are publicly available
as version 5.0 of Starburst99 at http://www.stsci.edu/science/starburst99/.
