The composition, metabolism, and significance of fatty acids in insect biology are addressed. Fatty acids enter a number of metabolic pathways not directly related to energy storage and production; the unifying theme is that the fatty acids are not only structurally altered in these pathways, but that the alterations carry them from one area of biological significance into another. This theme is developed by offering a perspective on fatty acids in insects and then reviewing three major areas: 1) fatty acid composition, 2) biosynthesis of fatty acids (including polyunsaturated fatty acids and characteristics of certain biosynthetic enzymes), and 3) the biological significance of fatty acids. This last section includes discussions of the biochemistry of waxes, pheromones, and prostaglandins and the roles of fatty acids as components of defensive secretions. Little is known at the biochemical level about the regulation of fatty acid metabolism, and it is suggested that work in this area represents another frontier in insect biochemistry.