Star clusters with a high central density contain an ecological network of evolving binaries, affected by interactions with passing stars, while in turn affecting the energy budget of the cluster as a whole by giving off binding energy. The energy liberated can balance the losses from the central regions by escaping stars as well as the energy lost by a heat flow toward the relatively colder cluster halo.

The `gravitational fusion' of single stars into binaries, triples, and collisional merger products proceeds via a complex reaction network. Although we are beginning to chart the activity in some of the major channels, there are still major uncertainties, and consequently our present knowledge of blue straggler formation and evolution is largely qualitative.