Joe Goldstein and Mike Brown have worked for over 30 years on the molecular basis of cholesterol homeostasis. Through the systematic use of genetics, biochemistry, molecular biology and cell biology, they have identified a complex set of interacting molecules that work coordinately to regulate cholesterol import and synthesis. Not only did they identify the crucial proteins in this pathway but also determined their function. An unexpected outcome of their work has been a new understanding of the structure and function of cell membranes. From the low-density lipoprotein receptor to sterol regulatory element binding protein (SREBP) to SREBP cleavage-activating protein to Insig-1, each protein has provided a new and fundamentally novel insight into how membranes function as molecular sensors that respond to changes in the metabolic condition of the cell by moving molecules between cellular compartments.^∗∗This article is part of the Pioneers series in Trends in Cell Biology. Pioneers articles feature researchers, experiments and concepts that forged the way to modern cell biology.