This chapter contains sections titled: Introduction Basic Organization Principles of a Cell Membrane Evidence for Phase Separation in Model Membrane Systems: Liquid-Ordered and Liquid-Disordered Phases Evidence for Phase Separation in Cell Membranes: The “Raft Concept” Raft Domains are Clustered to Exert their Function The Apical Membrane of Epithelial Cells: A Percolating Raft Membrane at 25 °C Caveolae: Scaffolded Membrane Domains Rich in Raft Lipids Caveolae and Lipid Rafts in Membrane Traffic