The muscles of lean fish contain about 0.7 g lipids/100 g, those of fish belonging to fatty species from several up to about 50 g/100 g, depending on the species and the feeding status. The lipids of lean fish are composed principally of phospholipids, while those of fatty fish are mainly triacylglycerols. The muscle lipids in fatty fish contain generally about 30 g/100 g of saturated fatty acids, 40 g/100 g monoenoic acids, and 30 g/100 g polyenoic fatty acids. The n-3 polyenoic fatty acids make up from several to about 50 g/100 g of the total amount of the polyenoic acids, depending on the lipid content in the fish meat. The lipids in refrigerated and frozen fish are hydrolyzed due to the activity of endogenous lipases and phospholipases. Fish lipids are highly susceptible to oxidation because of high content of polyenoic fatty acids. The rate of oxidation reactions is controlled by endogenous, enzymatic and non-enzymatic pro-oxidants and antioxidants of the fish muscle and skin. In smoked fish the highest antioxidant activity is displayed by smoke phenols. The loss of n-3 polyunsaturated fatty acids in smoked fish during storage is not known exactly, but it is probably not higher than about 1/10 of the initial content of these acids in the raw material.