Disulfide bond formation is part of the folding pathway for many periplasmic and outer membrane proteins that contain structural disulfide bonds. In Escherichia coli, a broad variety of periplasmic protein thiol:disulfide oxidoreductases have been identified in recent years, which substantially contribute to this pathway. Like the well-known cytoplasmic thioredoxins and glutaredoxins, these periplasmic protein thiol:disulfide oxidoreductases contain the conserved C-X-X-C motif in their active site. Most of them have a domain that displays the thioredoxin-like fold. In contrast to the cytoplasmic system, which consists exclusively of reducing proteins, the periplasmic oxidoreductases have either an oxidising, a reducing or an isomerisation activity. Apart from understanding their physiological role, it is of interest to learn how these proteins interact with their target molecules and how they are recycled as electron donors or acceptors. This review reflects the recently made efforts to elucidate the sources of oxidising and reducing power in the periplasm as well as the different properties of certain periplasmic protein thiol:disulfide oxidoreductases of E. coli.