The recent Keystone Symposium on Lipid Rafts and Cell Function (March 23-28, 2006 in Steamboat Springs, CO) brought together biophysicists, biochemists, and cell biologists to discuss the structure and function of lipid rafts. What emerged from the meeting was a consensus definition of a membrane raft: "Membrane rafts are small (10-200 nm), heterogeneous, highly dynamic, sterol- and sphingolipid-enriched domains that compartmentalize cellular processes. Small rafts can sometimes be stabilized to form larger platforms through protein-protein and protein-lipid interactions." This definition helps to clarify current thinking in a field that has been plagued by the heterogeneous and sometimes ephemeral nature of its subject.