Plant storage lipids are also important components of human and animal diets. Moreover, some are of particular interest for industry in the production of detergents, paints, and specialized lubricants. In all cases, their fatty acid composition determines their usefulness and economic value. Because of their economic importance, extensive information is available on the fatty acid composition and structure of a multitude of seed oils. However, much less was known about the biosynthesis of these oils. Triacylglycerols accumulate preferentially in seeds and fruits, although they can be found in all plant tissues. In seeds, the storage sites are the cotyledons or the endosperm. The accumulation of oil in seeds and fruits is a well characterized process. Several attempts have been made to modify seed oil fatty acid compositions using different genetic engineering techniques. Unusual fatty acids including medium-chain fatty acids, petro-selinic acid, and hydroxylated fatty acids are widely used by the chemical industry.