Lipoxygenase (EC 1.13.11.12, linoleate:oxygen oxidoreductase) is an iron-containing dioxygenase which catalyses the oxidation of polyunsaturated fatty acids and esters containing cis,cis-1,4 pentadiene system. The main reaction products are conjugated unsaturated fatty acids and hydroperoxides. The hydroperoxides generated by lipoxygenase are substrates for hydroperoxide lyase, hydroperoxide isomerase, peroxygenase and allene oxide synthase. The initial products of lipoxygenase activity may be degraded to variety of products, including several aldehydes, ketones and alcohols and characteristic aroma compounds. Lipoxygenases (LOX) may also catalyse the co-oxidation of carotenoids, including β-carotene, resulting in the loss of essential nutrients and the development of off-flavours. Lipoxygenase has been found in various organs of plants. Plant LOX proteins consist of a single polypeptide chain with a molecular mass about 75–100 kDa. The physiological role of lipoxygenase in plants is not certain, although there is considerable evidence indicating its involvement in wounding and other stress responses (drought and pest/pathogen attack) because lipoxygenases are believed to be on the biosynthetic pathways to the plants regulators abscisic acid and methyl jasmonate. Lipoxygenase activity is also the first step in the pathway leading to the formation of a number of flavour and aroma compounds. Lipoxygenases are of interest to food scientists because of their ability to form free radicals and peroxides which can be involved in indirect oxidation of vitamins, colours, phenolic and proteins.