Docosahaxaenoic acid and arachidonic acid are highly concentrated in the central nervous system. The amount of these fatty acids in the central nervous system increases dramatically during the last intrauterine trimester and the first year of life. A central question of research conducted during the past 20 years is if the essential fatty acid precursor of docosahexaenoic acid is sufficient to achieve optimal DHA accumulation in the central nervous system and, therefore, infant development. The important role of non-human primate studies in characterising the behavioral effects of n-3 essential fatty acid deficiency and subsequent low brain DHA accumulation, the difference between essential fatty acid deficiencies and conditional deficiencies of docosahexaenoic acid and arachidonic acid, and the evidence that human infants have a conditionally essential need for docosahexaenoic acid and, perhaps, for arachidonic acid are summarised. The current suggestive evidence for several possible mechanisms underlying behavioral effects are also provided.