Publisher Summary This chapter explains structural requirements, metabolism of action, and the role of the essential fatty acids. These acids are metabolized in ways common to other unsaturated fatty acids. In addition, the unsaturated acids possessing the 9, 12 double bond system (C18 acids) or the 11–14 double bond system (C20 acids) possesses special properties that are essential to the animal. Presumably, they function in esterified forms rather than as free fatty acids, and their essential roles may be determined by relative concentrations of phospholipids, glycerides, and sterol esters, which in turn seem to involve hormonal regulation. There is evidence to suggest that they are concerned, via phospholipids, in membrane integrity. This may be related to certain biochemical lesions that have been reported in connection with a deficiency of essential fatty acids in cases where membranes possess recognized biochemical activity, for example, in mitochondria. The essential fatty acids are the precursors of the prostaglandins, but the relationships of the latter to the classical deficiency symptoms is not certain and there is no evidence suggesting a relationship of this hormone family to the reported biochemical lesions of the deficiency.