The biological kingdoms have evolved elaborate systems that ensure the catalysis of protein disulfide bond (dsb) formation in the cell. Coexisting in the periplasm of Escherichia coli are the DsbA–DsbB disulfide-introducing and DsbC–DsbD disulfide-isomerizing pathways, which promote the oxidative folding of secreted proteins. Recent structural studies of DsbB have illuminated conformational dynamics involved in the effective oxidation of the extremely reduction-prone oxidase, DsbA, as well as the structure of the reaction centre involved in protein Dsb formation de novo in conjunction with ubiquinone. Extensive genetic and biochemical analysis has recently provided insight into how DsbD transports electrons from cytosolic thioredoxin to periplasmic DsbC. To a great extent, the molecular mechanisms of the Dsb enzyme system in E. coli have been elucidated, and are applicable to the study of protein disulfide formation systems in other organisms.