In this review (which is similar to part of the review that I presented at the IAU meeting 266 Star Clusters - Basic Galactic Building Blocks throughout Time and Space at he GA in Rio 2009) 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 3 specific topics: 1. the formation (via binaries) and evolution of very massive stars in dense clusters (like MUG 11 in M82) and the importance of stellar wind mass loss. 2. 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 3. the possible effect of massive binaries on the self-enrichment of globular clusters.