Population and spectral synthesis: it doesn't work without binaries
  In this chapter we discuss the population and spectral synthesis of stellar
populations. We describe the method required to achieve such synthesis and
discuss examples where inclusion of interacting binaries are vital to
reproducing the properties of observed stellar systems. These examples include
the Hertzsprung-Russell diagram, massive star number counts, core-collapse
supernovae and the ionising radiation from stellar populations that power both
nearby HII regions and the epoch of reionization. We finally offer some
speculations on the future paths of research in spectral synthesis.
