Eukaryotic cells possess several mechanisms for repairing double-stranded breaks in DNA. One mechanism involves genetic recombination with an intact sister duplex. The recent identification of the RAD51 protein, a eukaryotic homologue of Escherichia coli RecA, represents a landmark discovery in our understanding of the key reactions in this repair pathway. RAD51 is similar to RecA, both biochemically and structurally: it promotes homologous pairing and strand exchange within a regular nucleoprotein filament. The isolation of yeast and human RecA homologues shows that homologous recombination and recombinational repair have been conserved throughout evolution. The goal is now to identify other factors involved in recombinational repair and to define their roles in this essential process.