This chapter considers the structure of biological membranes and the organization of their two main constituents: lipids and proteins. It focuses mainly on the plasma membrane. The most abundant lipids in cell membranes are the phospholipids, which have a phosphate-containing, hydrophilic head linked to a pair of hydrophobic tails. Molecules with both hydrophilic and hydrophobic parts are termed amphipathic, a property shared by other types of membrane lipids, including the cholesterol, which is found in animal cell membranes and the glycolipids, which have sugars as part of their hydrophilic head. Many of the proteins and some of the lipids exposed on the surface of cells have attached sugar chains, which form a carbohydrate layer that helps, protect and lubricate the cell surface, while also being involved in specific cell-cell recognition. The chapter looks at the lipid bilayer, which constitutes the fundamental structure of all cell membranes.