Our understanding of the physiological function of insect essential lipids has long been flawed by major uncertainties. It was discovered long ago that dietary sterol is a necessary nutrient for all insects, which radically sets them apart from the vertebrates in terms of qualitative nutrient requirements. Because of the physiological importance of sterol as a molting hormone precursor in insects and the implications of this for the development of new insecticides, a wealth of investigation into insect sterol metabolism followed, covering both the ways in which insects convert diverse food-plant sterols into the major tissue sterols and how these in turn are metabolized into the ecdysone molting hormones. However, for the classes or essential Lipid nutrients required by vertebrates, research dealing with insects has been scant and, more often than not, rather indeterminate. Many, but by no means all, insects studied appear to require essential fatty acids, though virtually nothing has been found out about the metabolism or essential physiological function of these acids. Excepting vitamin A, needed for insect vision, the various vertebrate fat-soluble vitamins appear to have no significance for insect physiology, and results of the occasional attempts to demonstrate functions for them in growth and development have in most cases been tantalizingly equivocal. In recent years some notable advances were made in tne study or essential fatty acids and fat-soluble vitamins in insects, and work on insect sterol nutrition and metabolism continues with ever-increasing sophistication. The contributors to this book summarize, discuss, and speculate on these issues. Their work is based on papers presented at the 1980 World Congress of Entomology at Kyoto, Japan.