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.

Binaries are the most important energy reservoir of star clusters. Via three-body encounters, binaries can reverse the core collapse and prevent a star cluster from reaching equiparition. Moreover, binaries are essential for the formation of stellar exotica, such as blue straggler stars, intermediate-mass black holes and massive black hole binaries.

Color-magnitude diagrams of open clusters reveal many stars that do not fall on cluster main sequences or red giant branches including blue straggler stars, yellow giants, and sub-subgiants. In fact, as many as a quarter of the evolved stars in older open clusters do not fall on standard single-star isochrones. Rather than being anomalies, these stars are following frequently travelled alternative paths of stellar evolution. Most of these stars are in binary systems, and their origins likely stem from mass transfer, mergers and collisions within binaries. This chapter presents an overview of recent observational and modelling work to understand the processes that shape these alternative stellar evolution pathways, including an HST study of the blue straggler population of NGC 188, an abundance study of the blue stragglers of NGC 6819, establishing yellow giants as evolved blue straggler stars using asteroseismology, exploration of a new class of stars known as sub-subgiants, rotational identification of main sequence blue stragglers with Kepler/K2 and new insights into the angular momentum evolution of blue stragglers.