Liquid-liquid phase separation is common in aqueous macromolecule solutions, leading to formation of droplets that are greatly enriched in macromolecules. This presentation will describe studies of liquid-liquid phase separation in relatively simple polymer solutions, both as models for membraneless organelles in eukaryotic cells and as potential prebiotic compartments (membraneless protocells). Solutes such as ions, small molecules, and biopolymers can become compartmentalized by partitioning due to the different solvent environment and/or affinity interactions (e.g. ion pairing, hydrogen bonding). We are studying mechanisms for, and consequences of, this type of compartmentalization using a variety of simple model systems composed of phase-separating aqueous polymer solutions. Through these types of studies, we hope to uncover underlying physiochemical mechanisms in cellular organization and to identify new avenues for biomimetic systems for applications in biotechnology and materials science. For example, compartmentalization of catalysts and/or reactants into polymer-rich droplets can lead to control over the sites and rates of reactions. We have used this approach to increase ribozyme reaction rates and to develop liposome-stabilized water-in-water emulsions that act as artificial mineralizing vesicles. We have also studied the charge-mediated interaction of RNAs with cationic peptides as a means to generate simple model liquid organelles that can reversibly compartmentalize biomolecules. Formation and dissolution of these liquid bodies can be controlled by changes in peptide phosphorylation state using a kinase/phosphatase enzyme pair.