Some of the abundant data on the structure of biological membranes are compiled in this short review, with particular emphasis on morphological techniques. The "fluid mosaic" model of biological membranes has rapidly gained importance as the fundamental concept. According to this model, protein particles swim in a lipid bilayer, whose molecules are ordered with their hydrophilic end outside and their hydrophobic tail inside the membrane. Proteins are spatially stabilized and actively moved by a submembranous net of fibrillar proteins. Information about the orientation of protein molecules within the bilayer is obtained by protein labelling with membrane impermeable compounds and subsequent gel-electrophoretic separation of the solubilized components. Freeze etching brings to light intramembranous particles which are probably segments of the membrane proteins that stand out from the plane of the split hydrophobic membrane interface. By this method the morphology of cell contacts, for example, can be analyzed and comprehended at a supramolecular level. Special techniques such as observation of both matching sites of the split membrane halves or deep etching (whereby the actual surface of membrane is exposed) may provide further valuable contributions to the rapidly expanding field of membranology.