Disulfide bonds in proteins play various important roles. They are either formed as structural features to stabilize the protein or are found only transiently as part of a catalytic or regulatory cycle. In vivo, the formation and reduction of disulfide bonds is catalyzed by specialized thiol-disulfide exchanging enzymes that contain an active site with the sequence motif Cys-X-X-Cys. These proteins have structurally evolved to catalyze predominantly either oxidative reactions or reductive steps. There is mounting evidence that, in addition to the thiol redox potential, the spatial distribution within different cell compartments and the overall redox state of the cell are equally important. In the cytoplasm, multiple pathways play overlapping roles in the reduction of disulfide bonds and additionally, the expression of several components of thiol-redox pathways was shown to respond to the changes in the cellular thiol-redox equilibrium. In the periplasm, two systems coexist, one catalyzing thiol oxidation and the other disulfide reduction. Recent results suggest that two different mechanisms are used to translocate reducing power from the cytoplasm or to dissipate the electrons after oxidation.