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Fatty acids required for biological processes but does not include the fats that only act as fuel

Essential fatty acids, or EFAs, are fatty acids that humans and other animals must ingest because the body requires them for good health, but cannot synthesize them. Only two fatty acids are known to be essential for humans: alpha linolenic acid (an omega 3 fatty acid) and linoleic acid (an omega 6 fatty acid). These are supplied to the body either as the free fatty acid, or more commonly as some glyceride derivative. Deficiency in these fatty acids is rare. These fatty acids are essential because they are precursors to vitamins, cofactors, and derivatives, including prostaglandins, leukotrienes, thromboxanes, lipoxins, and others.

Fatty acids comprise an aliphatic hydrocarbon chain plus a carboxyl group (–COOH) at one end, and terminated by a methyl group (–CH3) at the other end. They are almost always straight chained. The carbon next to the carboxylate is known as α, the next carbon β, and so forth. Since biological fatty acids can be of diverse lengths, the last position is often labelled as a "ω", the last letter in the Greek alphabet. Thus, the ω 3 indicates that the first unsaturated carbon carbon bond from the terminal end (ω) of the chain is the third one. Typically, the number of carbons and the number of double bonds are also listed in short descriptions of unsaturated fatty acids. For instance, ω 3 18:4, or 18:4 ω 3, or 18:4 n−3 indicates stearidonic acid, an 18 carbon chain with 4 double bonds, and with a double bond between the third and fourth carbon atoms from the CH3 end. Double bonds are cis and separated by a single methylene (CH2) group unless otherwise noted. In free fatty acid form, the chemical structure of stearidonic acid is:

Some of the food sources of Ω 3 and Ω 6 fatty acids are fish and shellfish, seaweed oil, flaxseed (linseed) and flaxseed oil, hemp seed, olive oil, soya oil, canola (rapeseed) oil, chia seeds, pumpkin seeds, sunflower seeds, leafy vegetables, and walnuts. Essential fatty acids play a part in many metabolic processes, and there is evidence to suggest that low levels of essential fatty acids, or the wrong balance of types among the essential fatty acids, may be a factor in a number of illnesses, including osteoporosis. Fish is the main source of the longer omega 3 fats; eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), though they initially acquire these fats through the consumption of algae and seaweed. Some plant based foods contain omega 3 in the form of alpha linolenic acid (ALA), which appears to have a modest benefit for cardiovascular health. The human body can (and in case of a purely vegetarian diet often must unless certain algae or supplements derived from them are consumed) convert ALA to EPA and subsequently DHA. This elongation of ALA is inefficient. Conversion to DHA is higher in women than in men; this is thought to reflect the need to provide DHA to the fetus and infant during pregnancy and breast feeding. The IUPAC Lipid Handbook provides a very large and detailed listing of fat contents of animal and vegetable fats, including ω 3 and 6 oils. The National Institutes of Health's EFA Education group publishes Essential Fats in Food Oils. This lists 40 common oils, more tightly focused on EFAs and sorted by n 6:3 ratio. Vegetable Lipids as Components of Functional Food lists notable vegetable sources of EFAs as well as commentary and an overview of the biosynthetic pathways involved. However, these sources are not in perfect agreement. EFA content of vegetable sources varies with cultivation conditions. Animal sources vary widely, both with the animal's feed and that the EFA makeup varies markedly with fats from different body parts.

Essential fatty acids play an important role in the life and death of cardiac cells. Additionally, essential fatty acids are crucial for the development of several endocannabinoids with a multitude of functions in the body, such as Docosahexaenoyl ethanolamide (DHA EA/synaptamide). Many omega 3 and omega 6 derived essential fatty acids act similarly to endocannabinoids such as anandamide and 2 AG, possessing activity at the CB1 and CB2 receptors, among others. 29

Essential fatty acid deficiency results in a dermatitis similar to that seen in zinc or biotin deficiency.