Psychrophilic microorganisms, hosts of permanently cold habitats, produce enzymes which are adapted to work at low temperatures. When compared to their mesophilic counterparts, these enzymes display a higher catalytic efficiency over a temperature range of roughly 0-30 degrees C and a high thermosensitivity. The molecular characteristics of cold enzymes originating from Antarctic bacteria have been approached through protein modelling and X-ray crystallography. The deduced three-dimensional structures of cold alpha-amylase, beta-lactamase, lipase and subtilisin have been compared to their mesophilic homologs. It appears that the molecular adaptation resides in a weakening of the intramolecular interactions, and in some cases in an increase of the interaction with the solvent, leading to more flexible molecular edifices capable of performing catalysis at a lower energy cost.