Though under most circumstances harmful changes are induced in cellular structures by subzero temperatures, conditions can be found under which such damage is avoided. Thus, in solution, biochemical reactions can be slowed and more easily analysed and many enzyme-substrate complexes can be stabilized and structurally analysed; in crystals, `stop-action' pictures unveil the stereochemical changes along reaction pathways. The progressive `solidification' of non-covalent bonds involved in protein structures should permit investigation of their dynamics. Studies at high pressures open the way to new investigations on `activated' enzyme-substrate complexes and might permit the refinement of current concepts to a considerable degree, as a preliminary but decisive step towards a full description of enzyme mechanisms. The conditions of medium allowing such cryobiochemical studies fail to protect cellular structures against cold. Investigations of plasma membrane behaviour are now under way to determine processes leading to cryosensitivity or cryotolerance of cells.