Horizontal DNA transfer among bacteria (conjugation) requires the formation of secure intercellular contacts before DNA transfer can occur. The formation of such contacts among Gram-negative bacteria is mediated by conjugative pili. Like other pili, conjugative pili are filaments extending from the surface of donor cells. They are composed, in so far as is known, entirely of conjugative pilin subunits. Here, we review the structure of F-pilin, the subunit of which F-pili are composed. We emphasize recent studies suggesting a specific domain organization for F-pilin and present a model of how these domains might be arranged in filament subunits.