Docosahexaenoic acid (DHA) is a 22-carbon essential fatty acid with 6 double bonds. In the brain, DHA modulates a number of important neurochemical processes such as neuro transmitter release, gene expression, immunity and neuroinflammation, learning and memory, apoptosis, and activities of membrane-bound enzymes, ion channels, and receptors. In the retina, DHA-enriched phospholipids are highly concentrated in the immediate environment of rhodopsin and play an important role in visual transduction. Although details of the molecular mechanisms remain unknown, DHA incorporation into neural membrane phospholipids not only alters their physicochemical properties, but also markedly affects the signal transduction processes associated with optimal brain function. Deficiency of docosahexaenoic acid is associated with Alzheimer disease, stroke, hyperactivity, schizophrenia, and peroxisomal disorders. It is remarkable to note that this fatty acid participates in so many neurochemical processes and neurological diseases. DHA supplementation restores gene expression and modulates neurotransmission, and signal transduction processes associated with behavioral deficits, learning activity and psychotic changes in schizophrenia, depression, hyperactivity, stroke, Alzheimer disease, and peroxisomal disorders.