Close pairs: keys to comprehension of the evolution of star clusters

In this review I first summarize why binaries are key objects in the study of stellar populations, key objects to understand the evolution of star clusters, key objects to understand galaxies and thus the universe. I then focus on 4 specific topics: 1. the formation (via 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 2. the evolution of intermediate mass binaries in a starburst with special emphasis on the variation of the SN Ia rate (the delayed time distribution of SN Ia). A comparison with SN ia rates in elliptical galaxies may provide important clues on the SN Ia model as well as on the evolution of the SN Ia progenitors 3. the evolution of the double neutron stars mergers in a starburst (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 4. the possible effect of massive binaries on the self-enrichment of globular clusters.