**Abstract**

Fatty acid constitutes the foundation cell membranes, provides metabolic energy, affects functions of membrane-bound enzymes/receptors, conducts signaling cascades, and helps in learning-related memory cognition in mammals, including humans. Structurally, the fatty acids are of two kinds: saturated and unsaturated; the latter are again of mono- and polyunsaturated types. From nutritional perspectives, they are of essential and nonessential types. Omega-6 linoleic acid (ω-6 LLA, C18:2) and ω-3 alpha linolenic acid (ω-3 αLLN, C18:3) and ω-6 arachidonic acid [(ω-6 AA, C20:4); it is conditional] are essential fatty acids (EFAs). In addition, mammalian brains cannot biosynthesize the ω-3 docosahexaenoic acid (ω-3 DHA, C22:6) in adequate amounts because of lack of necessary enzymes. Thus, DHA is essential for the growth and development of the brains. Deficiency of DHA produces visual- and learning-related memory impairments, and neurodegeneration in the aged brains and Alzheimer's disease brains. Finally, this chapter will highlight and broaden the awareness about the essentiality of different fatty acids with a special emphasis on DHA.

**Keywords:** docosahexaenoic acid, eicosapentaenoic acid, arachidonic acid, alpha-linolenic acid and linoleic acid, eicosanoids, docasonoids, brain cognition
