Close pairs: keys to comprehension of star cluster evolution

In this review I first summarize why binaries are key objects in the study of stellar populations, to understand the evolution of star clusters and galaxies, and thus to understand the universe. I then focus on four specific topics:(i) the formation (through binaries) and evolution of very massive stars in dense clusters and the importance of stellar-wind mass loss. I discuss preliminary computations of wind mass-loss rates of very massive stars performed with the Munich hydrodynamical code and the influence of these new rates on the possible formation of an intermediate-mass black hole in the cluster MGG 11 in M82;(ii) the evolution of intermediate-mass binaries in a starburst with special emphasis on the variation of the supernova (SN) Ia rate (i.e., on the delayed time distribution of SNe Ia). A comparison with SN Ia rates in elliptical galaxies may provide important clues to SN Ia models as well as to the evolution of SN Ia progenitors;(iii) the evolution of double-neutron-star mergers in a starburst (i.e., the delayed time distribution of these mergers) and what this tells us about the suggestion that these mergers may be important production sites of r-process elements;(iv) the possible effect of massive binaries on the self-enrichment of globular clusters.