Continuous cold perfusion by machine combines the benefits of hypothermia with continuous buffering, continuous washout of accumulating toxic metabolites, and continuous provision of oxygen and nutrients. Oxygen is more soluble at lower temperatures, and the reduced energy requirement of hypothermic organs can be provided by acellular perfusates. For all organs studied, viability has been maintained longest by continuous machine perfusion. A heat exchanger is needed to maintain temperature at, an atraumatic pump circulates the solution, and oxygenation is achieved either by simple surface diffusion or using a membrane oxygenator. An oxygen/carbon dioxide gas mixture, and an effective buffer are required to maintain a constant pH. A bubble trap and organ chamber complete the circuit. Materials coming in contact with the fluid must be sterile, non-toxic plastics or metals. The perfusate must contain colloid to maintain intravascular volume and prevent development of an extracellular fluid space. Cryoprecipitated plasma or albumin was initially used, but albumin gradually leaks into the interstitium and leads to weight gain, so this has been replaced by other oncotic colloids (gelatin, dextrans, hydroxyethyl starch). Metabolic fuels improve long-term preservation—glucose has been the substrate most commonly used, while adenosine, other precursors, or ATP itself have been used as additional sources of energy. Organ reperfusion requires rapid renewal of membrane pump activity, and quick regeneration of cellular fuels. Addition of other substrates more specific to individual organs can also improve function after storage (arachidonic acid, essential amino acids, fatty acids). Fuel additives become increasingly important as preservation times are prolonged beyond 24 h.