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 both 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.

When the two EFAs were discovered in 1923, they were designated "vitamin F", but in 1929, research on rats showed that the two EFAs are better classified as fats rather than vitamins.