The population of hot stars is very diverse, ranging from young
massive main sequence stars, giants and supergiants to old and
evolved objects like white dwarfs and their immediate
progenitors. Studying those stars helps us to understand key problems
stellar astrophysics like the formation of massive stars, close binary
interactions, radiation-driven winds, and the final stages of
stellar evolution. Solving these problems is central to our
understanding of related issues like supernovae, with important
implications for cosmology, and the history of chemical enrichment in
galaxies. Although considerable progress has been made in the
last decade, analysing hot stars remains challenging, because peculiar
chemical compositions, NLTE effects, mass-loss, rotation,
variability, and binarity have to be taken into account.
Trying to make sense out of the observational evidence makes us aware
of the shortcomings in stellar evolution models.