  We present a new set of synthesis models for stellar populations obtained
with Starburst99, which are based on new stellar evolutionary tracks with
rotation. We discuss models with zero rotation velocity and with velocities of
40% of the break-up velocity on the zero-age main-sequence. These values are
expected to bracket realistic rotation velocity distributions in stellar
populations. The new rotating models for massive stars are more luminous and
hotter due to a larger convective core and enhanced surface abundances. This
results in pronounced changes in the integrated spectral energy distribution of
a population containing massive stars. The changes are most significant at the
shortest wavelengths where an increase of the ionizing luminosity by up to a
factor of 5 is predicted. We also show that high equivalent widths of
recombination lines may not necessarily indicate a very young age but can be
achieved at ages as late as 10 Myr. Comparison of these two boundary cases (0
and 40% of the break-up velocity) will allow users to evaluate the effects of
rotation and provide guidance for calibrating the stellar evolution models. We
also introduce a new theoretical ultraviolet spectral library built from the
Potsdam Wolf-Rayet (PoWR) atmospheres. Its purpose is to help identify
signatures of Wolf-Rayet stars in the ultraviolet whose strength is sensitive
to the particulars of the evolution models. The new models are available for
solar and 1/7th solar metallicities. A complete suite of models can be
generated on the Starburst99 website (www.stsci.edu/science/starburst99/). The
updated Starburst99 package can be retrieved from this website as well.
