The foregoing review of the structure, physical properties and function of membrane lipids in archaebacteria has revealed a remarkable variety of polar lipid classes, including phospholipids, glycolipids, phosphoglycolipids and sulfolipids, all derived from the one basic core structure, diphytanylglycerol (I). Even with the relatively limited knowledge that we have of the physical properties of these lipids, it is clear that they are well-adapted as membrane components to the particular environmental conditions of the three groups of archaebacteria, extreme halophiles, methanogens, and thermoacidophiles. However, much remains to be learned concerning the precise asymmetric arrangement of the lipids in the membrane bilayers or monolayers, the interaction of the lipids with the membrane proteins, and the function of this membrane lipid asymmetry with respect to ion transport, permeability to nutrients, proton transport and conductance, and energy transduction. Perhaps then these unusual lipids will not appear so strange and our knowledge of them will help us understand the function of the more familiar lipids in the eubacteria and eukaryotes.