Mutations in BRCA2 increase susceptibility to breast, ovarian and prostate cancers. The product of human BRCA2, BRCA2 protein, plays a key role in the repair of DNA double strand breaks and interstrand crosslinks by RAD51-mediated homologous recombination. Here, we present a biochemical and structural characterization of full length (3,418 amino acid) BRCA2, alone and in complex with RAD51. We show that BRCA2 facilitates nucleation of RAD51 filaments at multiple sites on single-stranded DNA. Three-dimensional electron microscopy reconstructions revealed that BRCA2 exists as a dimer and that two oppositely-oriented sets of RAD51 molecules bind the dimer. Single stranded DNA binds along the long axis of BRCA2, such that only one set of RAD51 monomers can form a productive complex with DNA and establish filament formation. Our data define the molecular mechanism by which this tumor suppressor facilitates RAD51-mediated homologous recombinational repair.