In view of the high omega-3 poly unsaturated fatty acid content of the brain, it is evident that these fats are involved in brain biochemistry, physiology and functioning; and thus in some neuropsychiatric diseases and in the cognitive decline of ageing. Though omega-3 fatty acids (from fatty fish in the human diet) appear effective in the prevention of stress, their role as regulator of mood and of libido is a matter for discussion pending experimental proof in animal and human models. Dietary omega-3 fatty acids play a role in the prevention of some disorders including depression, as well as in dementia, particularly Alzheimer's disease. Their direct role in major depression, bipolar disorder (manic-depressive disease) and schizophrenia is not yet established. Their deficiency can prevent the renewal of membranes, and thus accelerate cerebral ageing; none the less, the respective roles of the vascular component on one hand (where the omega-3's are active) and the cerebral parenchyma itself on the other, have not yet been clearly resolved. The role of omega-3 in certain diseases such as dyslexia and autism is suggested. In fact, omega-3 fatty acids participated in the first coherent experimental demonstration of the effect of dietary substances (nutrients) on the structure and function of the brain. Experiments were first of all carried out one x-vivo cultured brain cells (1), then on in vivo brain cells(2), finally on physiochemical, biochemical, physiological, neurosensory, and behavioural parameters (3). These findings indicated that the nature of poly unsaturated fatty acids(in particular omega-3) present in formula milks for infants (both premature and term) determines the visual, cerebral,and intellectual abilities, as described in a recent review (4). Indeed,the insufficient dietary supply of omega-3 fatty acids in today's French and occidental diet raises the problem of how to correct dietary habits so that the consumer will select foods that are genuinely rich in omega-3/ the omega-3 family ; mainly rapeseed, (canola) and walnut oils on one hand and fatty fish on the other.