Abstract: 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. I will present the results of projects that address the lipidomic, biophysical, and functional aspects of mammalian plasma membranes. These projects explore the relationship between membrane organization and cellular function, ultimately demonstrating that membrane phenotypes are central regulators of cell physiology.