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Lipids supply essential fatty acids (EFA) and energy in fish diets. Most fish cannot synthesize (de novo) polyunsaturated fatty acids (PUFA) and therefore they must be supplied in the diet for normal growth, reproduction and health. EFA include PUFA of the n-3 and n-6 series, e.g. α-linolenic acid, 18:3n-3 and linoleic acid, 18:2n-6. Generally, EFA requirements of freshwater fish can be met by the supply of 18:3n-3 and 18:2n-6 fatty acids in their diets. By contrast, the EFA requirement of marine fish can only be met by supplying the correct concentrations and ratios of the long-chain PUFAs, eicosapentaenoic acid (20:5 n-3; EPA) and docosahexaenoic acid (22:6n-3; DHA) with perhaps some arachidonic acid (20:4n-6; AA), a highly unsaturated member of the n-6 series. (NRC, 1993; Higgs and Dong, 2000). Freshwater fish are able to elongate and desaturate 18:3n-3 to 22:6n-3, whereas marine fish, which lack or have a very low activity of 5-desaturase, require the long-chain PUFAs, mainly from the n-3 series. This presentation will briefly review the status of knowledge on the relationship between EFA and immune functions with emphasis on eicosanoi