Fatty acids are directly involved in the structure of most lipids, including those in the nervous system, providing their chemical and biological characteristics. Besides saturated and monounsaturated fatty acids, two families of polyunsaturated fatty acids, the omega-6 fatty acids, such as linoleic acid (LA), and the omega-3 fatty acids, such as alpha linolenic acid (ALA), are most important. LA and ALA are essential nutrients, as the human body cannot synthesize them or convert one to the other. They were formerly called vitamin F, before their chemical structures were determined. The term “omega-3 fatty acids” is plural as there are four main ones that have increasing numbers of double bonds and carbon atoms. ALA (18:3 ω3) is the precursor of stearidonic acid (18:4 ω3, SA), which gives rise to eicosapentaenoic acid (EPA, 20:5 ω3) and docosapentaenoic acid (DHA, 22:6 ω3). The omega-3 polyunsaturated fatty acid content of the brain is extremely high, indicating that these fats are involved in brain physic chemistry, biochemistry, physiology and function; and, consequently, in brain development, in some neuropsychiatric diseases and in the cognitive decline of aging. This review examines all three aspects Some studies on perinatal cerebral development have focused on ALA, whereas others have examined long-chain derivatives, DHA and, to a lesser extent EPA. A third group of studies has examined the influence of ALA and DHA, sometimes with the omega-6 fatty acid, arachidonic acid (ARA). The studies on ALA provided the first demonstration of the effect of a dietary component on the structure and function of the brain that involved several scientific disciplines. These included cultures of dissociated brain cells; analyses of the fatty acids and lipids and cell types in the brain; regions and classes of phospholipids; physicochemical studies on brain membrane fluidity; biochemical and enzymological studies on enzymes such as ATPase; physiological studies on dopaminergic, serotoninergic, and cholinergic neurotransmission; toxicology of heavy metals and trans fatty acids; studies on vision, hearing, and taste; electrophysiological studies (ERG and EEG); and cognitive and behavioral studies, memory and habituation being specifically affected. The accumulation of considerable experimental evidence led to the inclusion of ALA in baby formulas. This decision has been confirmed by many studies on newborns. The nature of the polyunsaturated fatty acids (particularly the omega-3 fatty acids) in baby formulas for both full term and premature infants influences the infant's visual, neurological, cerebral, and intellectual capacities. Enrichment with long-chain fatty acids such as DHA (and EPA) is based on the fact that human milk contains them, unlike animal milks, that the brain is extremely rich in these fatty acids, and there is little desaturase activity (which, together with elongases, transforms ALA to DHA). Clinical studies on EPA and DHA, both found in fish oils, preceded or paralleled those on animals, in contrast to the work on ALA. Despite a lack of exhaustive experiments, DHA and EPA were added to baby formulas, which may well have a limited or even negative effect because of competition with omega-6 fatty acids. Formula supplemented with DHA and ARA has a positive effect on membranes, including nerve membranes, on physiological, electrophysiological, and sensory parameters, particularly vision and, most recently, on hearing. However, there is still debate about their influence on motor function, neurological capacity, behavior, and cognition. In fact, both DHA and ALA are essential fatty acids because of the low desaturase activities in the brain and liver. Dietary omega-3 fatty acids from oily fish in the human diet (the efficiency of fish oil in capsule has not yet been clearly demonstrated) may help prevent some psychiatric disorders, mainly depression and dementia, particularly Alzheimer's disease. The direct role of omega-3 fatty acids in bipolar disorder (manic-depressive disease) and schizophrenia has not yet been clearly established, although suspected in some aspects of these diseases. Omega-3 fatty acids may also be involved in dyslexia and autism, but there is evidence that they appear to help prevent stress. Finally, their influence on mood and libido is a matter for discussion pending experimental proof in animals and humans. A lack of dietary omega-3 fatty acid can prevent the renewal of membranes, and thus accelerate cerebral aging, but the roles of the vascular system (where the omega-3 fatty acids are active) and the cerebral parenchyma itself have not yet been clearly resolved. The insufficient dietary supply of omega-3 fatty acids (mainly ALA) in today's occidental diet (and in a number of diets throughout the world) raises the problem of how to correct dietary habits so that the consumer selects foods that are rich in omega-3 fatty acids; mainly rapeseed (canola) and walnut oils, oily fish, and certain eggs. Alpha tocopherol protects the omega-3 fatty acids in the nervous system membranes and not other components of vitamin E.