The large enzyme families of protein disulfide isomerases and peptidyl prolyl cis/trans isomerases have been shown to assist polypeptide restructuring. Various folding states of polypeptides may serve as substrates of the catalysed reaction. Our understanding of the cellular function of these enzymes is increasing as a result of the availability of more specific inhibitors, the discovery of natural substrates and the use of genetically modified organisms. Further highlights of these studies include insights into the three-dimensional structures of enzyme-ligand complexes, as well as into the mechanism of slow folding phases on the atomic level.