Since this is the first review on lipids to appear in the Annual Review of Physiology, it should cover the spectrum of the functional roles of lipids: (a) as fuel; (b) as membrane components; (c) in blood coagulation; (d) as components of immune systems; (e) as vitamins and hormones; and (j) in interfacial phenomena (e.g. in digestive processes and at the gas-liquid interface of lung alveoli). Space limitations make it impossible to consider all of these areas. Fortunately, the latter four have been the subject of many reviews, not as lipids per se but as components of physiologi�al systems. Therefore this discussion is devoted to two of the functions: as fuel ,lU ld as membrane components. The following review articles are offered as a guide to the other four areas: blood coagulation (110, 204); immune systems (157); hormones and vitamins (21, 39, 55, 60, 84, 103, 154, 203); interfacial phenomena in digestive processes (15, 16); lung surfactant (91, 127, 195).