Docosahexaenoic acid (DHA) is a long-chain, highly unsaturated omega-3 (n-3) fatty acid. It has a structure that gives it unique physical and functional properties. DHA is metabolically related to other n-3 fatty acids: it can be synthesised from the plant essential fatty acid α-linolenic acid (ALA). However, this pathway does not appear to be very efficient in many individuals, although the conversion of ALA to DHA is much better in young women than in young men. Furthermore, young infants may be more efficient converters of ALA to DHA than many adults, although the conversion rate is variable among infants. Many factors have been identified that affect the rate of conversion. The implication of poor conversion is that preformed DHA needs to be consumed. DHA is found in fairly high amounts in seafood, especially fatty fish, and in various forms of n-3 supplements. The amount of DHA in seafood and in supplements varies. Breast milk contains DHA. DHA is found esterified into complex lipids within the bloodstream, in adipose stores and in cell membranes. Its concentration in different compartments varies greatly. The brain and eye have high DHA contents compared to other organs. DHA is especially concentrated in the grey matter of the brain and in the rod outer segments of the retina. In the brain DHA is involved in neuronal signalling, while in the eye it is involved in the quality of vision. DHA is accumulated in the brain and eye late in pregnancy and in early infancy. A lower DHA content is linked to poorer cognitive development and visual function. DHA affects cell and tissue physiology and function through numerous mechanisms, including alterations in membrane structure and function, in membrane protein function, in cellular signalling and in lipid mediator production.