Solutions of structurally different macromolecules, when mixed, above certain concentrations, tend to phase separate. The high concentration and diversity of proteins present in the liquid phase of cytoplasm, together with the phenomena which accompany macromolecular crowding, appear to meet the requirements for multiphase separation. The resulting cytoplasmic phase compartments, bounded by interfaces and/or other intracellular surfaces, would provide a dynamic, three-dimensional organizational structure. Based on the known physicochemical properties of aqueous phase systems and the partitioning behavior of biomaterials in them, such an organization could account for numerous phenomena observed in cytoplasm, such as its microcompartmentation. Aqueous phase separation is an attractive model for the liquid phase of cytoplasm because it comprises not only a structure (phase compartments, interfaces) for constraining biomaterials but also a mechanism (partitioning) for translocating them to other sites.