AbstractUnsaturation in fatty acids follows definite patterns. In the mono‐ethylenic acids a nine‐carbon chain occurs frequently on one side or the other of the double bond, a fact probably significant in relation to the structure of oleic acid. The non‐conjugated polyenoic acids fall mainly into three series, showing affinities with oleic, linoleic and linolenic acid respectively when considered from the terminal methyl carbon. This may result largely from positional limitations during metabolism of these three acids by animals. Saturated and unsaturated fatty acids are treated selectively by enzymes when being esterified with glycerol, but this selectivity follows a different course in triglycerides and lecithins. Nevertheless, there appear to be some common features in the patterns of enzymic synthesis of these and some other lipids. Characteristic differences in the fatty acids of triglycerides, glycero‐phosphatides, sphingolipids and cholesterol esters are another feature of selective enzymic acylation, particularly intriguing because of dynamic equilibrium with a common metabolic pool of fatty acids. Many lipids, including those based on glycerol, sphingosine and phyto‐sphingosine, show structural and enzymic inter‐relationships.