Population Synthesis at the Crossroads
  The current state-of-the-art of population synthesis is reviewed. The field
is currently undergoing major revisions with the recognition of several key
processes as new critical ingredients. Stochastic effects can artificially
enhance or suppress certain evolutionary phases and/or stellar mass regimes and
introduce systematic biases in, e.g., the determination of the stellar initial
mass function. Post-main-sequence evolution is often associated with irregular
variations of stellar properties on ultra-short time-scales. Examples are
asymptotic giant branch stars and luminous blue variables, both of which are
poorly treated in the models. Stars rarely form in isolation, and the fraction
of truly single stars may be very small. Therefore, stellar multiplicity must
be accounted for since many systems will develop tidal interaction over the
course of their evolution. Last but not least, stellar rotation can drastically
increase stellar temperatures and luminosities, which in turn leads to revised
mass-to-light ratios in population synthesis models.
