Cell membranes contain regions called "rafts" in which particular lipids and proteins are concentrated. Recent literature has implicated raft domains in vital cell functions including budding, endocytosis, adhesion, signaling, and protein function. Given the biological significance of rafts, it is critical to understand the fundamental physical behavior underlying raft formation. With this goal, my research group studies free-floating giant unilamellar vesicles as models of cell membranes. We observe liquid domains in vesicles composed of lipids extracted from cells, as well as of simple ternary mixtures of lipids and cholesterol. We utilize fluorescence microscopy to identify miscibility transitions in the vesicle membranes, and to determine compositions of the resulting two liquid phases. We also couple fluorescence microscopy with other experimental methods to reveal llipid organization on different length scales. For example, we use NMR to determine tie-lines and free energies of lipid transfer between liquid phases. In experiments underway, we utilize electron microscopy to probe sub-micron domains. In order to assemble membranes with asymmetric leaflet compositions as in cells, we extend our techniques beyond lipid vesicles to supported membranes and monolayers.