Submersed cultures are increasingly being used for fermentation with animal cells. Reactor design is particularly important in these operations, because of the sensitivity of the cells to shear. In addition to the usual aeration methods, open-pore membranes or pure diffusion membranes are used for oxygenation in order to avoid gas bubbles. The various oxygenation methods are described in the present article [1]. Design principles for surface aeration, bubble columns, loop reactors, and stirred tanks, as well as oxygenation with Accurel or silicone membranes, are presented and discussed specifically for the low oxygen inputs desired in cell cultures. The scale laws are formulated, and special reference is made to problems of scale up. The various oxygenation methods are finally compared on the basis of the design principles presented, with particular attention to mechanical stress on the cells and to the laws of scale translation.