The so-called blue straggler Ssars (BSSs) represent the most striking (and certainly the most famous) evidence of binary stars in Galactic Globular Clusters (GCs). In this chapter, the most intriguing properties of BSSs are discussed and two innovative tools based on the physical properties of these fascinating objects are presented: (1) the dynamical clock, and (2) the stellar scale. The former uses the level of central segregation of BSSs to empirically measure the level of dynamical evolution suffered by the parent cluster. The the stellar scale is a spectroscopic tool that allows differential measures of stellar mass able to efficiently distinguish massive objects (as the elusive evolved BSS progeny) from normal low-mass cluster stars.

This chapter discusses the population and spectral synthesis of stellar populations. It describes the method required to achieve such synthesis and discusses examples where inclusion of interacting binaries are vital to reproducing the properties of observed stellar systems. These examples include the Hertzsprung–Russel 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. It finally offers some speculations on the future paths of research in spectral synthesis.