\documentstyle[12pt]{article}

\title{\bf Spectral Synthesis Analysis of Hot-Star Populations
\footnote{Based on observations with the NASA/ESA {\it Hubble Space 
Telescope}, obtained at
the Space Telescope Science Institute, which is operated by AURA for NASA
under contract NAS5-26555}}

\author{Claus Leitherer\\
\vspace{1cm}\\
\normalsize Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,
       MD 21218}
        
{\noindent\small{\bf Abstract:} 
Techniques to determine the high-mass star content of galaxies from their
integrated light are reviewed. The latest generation of hot star atmospheres
and stellar evolution models, in combination with empirical libraries, allow
the construction of detailed evolutionary synthesis models. The 
optical and the ultraviolet spectral regions are discussed. 
Comparison with observations provides constraints on the history of
high-mass star formation in galaxies. Wolf-Rayet stars play a crucial role
in setting the clock during a burst of star formation. The presence
of massive stars has been inferred from their spectral signatures in 
galaxies up to redshifts $z \approx 3$. A summary of key results is presented
and areas of major uncertainties are pointed out. }