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Cell membranes are dynamic assemblies of a variety of lipids and proteins. They form a protective layer around the cell, but also mediate the communication with the outside world – that is, neighboring cells in a tissue, hormones and growth factors arriving with the blood supply, or pathogens trying to enter the system. The unique feature of cell membranes is that their lipid and protein constituents can self-assemble into 5 nm-thin, two-dimensional fluids composed of two apposing lipid monolayers that form a hydrophobic interior and two polar interfacial regions oriented towards the aqueous medium. This organizing principle – the lipid bilayer – is the oldest, still valid molecular model of biological structures. The first model that incorporated proteins was proposed by Danielli and Davson, and assumed that the bilayer was made up entirely of lipids and that proteins covered the two polar surfaces [1]. Some 40 years later, the fluid mosaic model of the cell membrane proposed by Singer and Nicolson [2] was a conceptual breakthrough. Amphipathic membrane proteins were recognized to reside within, and even span, the whole bilayer that was depicted as a dynamic structure, the components of which are laterally mobile. However, the view that the lipids in the bilayer mainly serve as a homogeneous solvent for proteins [2] has been proven to be too simplistic. Lipids are not only distributed asymmetrically between the two leaflets of the bilayer, but also within the leaflet they are heterogeneously arranged [3]. This chapter will recapitulate the history and recent advances in membrane biology including the lipid raft concept, and then summarize current views on the functions of rafts and caveolae in membrane traffic.