Why should sexual reproduction be so widespread, despite its obvious disadvantages? It is arguable that sex between bacteria is an accidental side-effect of other processes. However, the elaborate cycle of meiosis and sexual union that is part of the life cycle of almost all eukaryotes is an adaptation - most likely, an adaptation for generating variation that will facilitate natural selection. This explanation seems intuitively obvious, yet it has taken several decades for its theoretical basis to become clear. Indeed, we do not yet know what kind of selection is involved (deleterious mutations, adaptive substitution, or coevolution between hosts and pathogen, ...), and we do not understand how this selection can maintain sexual reproduction despite its costs. The recent flood of genomic data may soon resolve these questions.