  With the coming generation of instruments and telescopes capable of
spectroscopy of high redshift galaxies, the spectral synthesis technique in the
rest-frame UV and Far-UV range will become one of a few number of tools
remaining to study their young stellar populations in detail. The rest-frame UV
lines and continuum of high redshift galaxies, observed with visible and
infrared telescopes on Earth, can be used for accurate line profile fitting
such as PV@1118,1128A, CIII@1176A, and CIV@1550A. These lines are very precise
diagnostic tools to estimate ages, metallicities, and masses of stellar
populations.
  Here we discuss the potential for spectral synthesis of rest-frame UV spectra
obtained at the Keck telescope. As an example, we study the 8 o'clock arc, a
lensed galaxy at z=2.7322. We show that the poor spectral type coverage of the
actual UV empirical spectral libraries limits the age and metallicity
diagnostic. In order to improve our knowledge of high redshift galaxies using
spectral synthesis, UV stellar libraries need to be extended to obtain accurate
age, metallicity, and mass estimates likely to be occuring in young stellar
populations observed in the early universe.
