  The spectral libraries of hot, massive stars which are implemented in the
population synthesis code Starburst99 are discussed. Hot stars pose particular
challenges for generating libraries. They are rare, they have an intense
radiation field and strong stellar winds, and a luminosity bias towards
ultraviolet wavelengths. These properties require the utilization of
theoretical libraries. Starburst99 uses static non-LTE models at 0.3 A
resolution in the optical, spherically extended, expanding models at 0.4 A
resolution in the satellite-ultraviolet, and blanketed, low-resolution
radiation-hydrodynamical models in the extreme ultraviolet down to X-rays. I
review the main features of each library, compare them to observations, and
discuss their link with stellar evolution models.
