  Rotation plays a major role in the evolution of massive stars. A revised grid
of stellar evolutionary tracks accounting for rotation has recently been
released by the Geneva group and implemented into the Starburst99 evolutionary
synthesis code. Massive stars are predicted to be hotter and more luminous than
previously thought, and the spectral energy distributions of young populations
mirror this trend. The hydrogen ionizing continuum in particular increases by a
factor of up to 3 in the presence of rotating massive stars. The effects of
rotation generally increase towards shorter wavelengths and with decreasing
metallicity. Revised relations between star-formation rates and monochromatic
luminosities for the new stellar models are presented.
