A crucial step in many biochemical studies is the separation and purification of the individual structures of interest. For cells, cell organelles, and membrane fragments, the use of aqueous two-phase partition has become an important tool. The basis for separation in these phase systems is the surface properties of the material, and the method, therefore, nicely complements the more traditional centrifugation techniques. Excellent separations have sometimes been obtained in just one or a few partition steps. However, in most instances, a single partition step only gives a partial or incomplete separation of components. By a multiple-extraction procedure, the resolving power can be drastically increased and components with only small differences in surface properties can be separated.