The plasma membrane is the interface between a cell and its environment, and is therefore responsible for a myriad of parallel processing tasks that must be tightly regulated to avoid aberrant signaling. To achieve this functional complexity, mammalian cells produce hundreds of lipid species that are actively turned over and trafficked to produce spatial and temporal gradients between cellular compartments. In addition to the plethora of regulatory roles performed by individual lipid molecules, membrane physiology is strictly dependent on the biophysical phenotypes – including membrane fluidity, rigidity, lipid packing, and lateral organization – arising from the collective behaviors of lipids. A key feature of metazoan membranes is their partitioning into functional lateral domains, e.g. membrane rafts. Although such domains have been implicated in a large variety of processes occurring at the plasma membrane, their properties, compositions, and precise functional roles in cells remain elusive. I will present the results of two distinct projects that attempt to address (1) the diverse lipid composition of cellular plasma membranes, how this composition is dependent on dietary fats, and how these determine the biophysical and signaling properties of cellular membranes; and (2) the structural determinants and functional consequences of protein association with ordered membrane domains. Both projects explore the relationship between membrane organization and cellular function, ultimately demonstrating that membrane phenotypes are central regulators of cell physiology.