Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are n‐3 fatty acids found in oily fish and fish oil supplements. These fatty acids are able to inhibit partly a number of aspects of inflammation including leucocyte chemotaxis, adhesion molecule expression and leucocyte‐endothelial adhesive interactions, production of eicosanoids like prostaglandins and leukotrienes from the n‐6 fatty acid arachidonic acid, production of inflammatory cytokines and T cell reactivity. In parallel, EPA gives rise to eicosanoids that often have lower biological potency than those produced from arachidonioc acid and EPA and DHA give rise to anti‐inflammatory and inflammation resolving resolvins and protectins. Mechanisms underlying the anti‐inflammatory actions of n‐3 fatty acids include altered cell membrane phospholipid fatty acid composition, disruption of lipid rafts, inhibition of activation of the pro‐inflammatory transcription factor nuclear factor kappa B so reducing expression of inflammatory genes, activation of the anti‐inflammatory transcription factor NR1C3 (i.e. peroxisome proliferator activated receptor γ) and binding to the G protein coupled receptor GPR120. These mechanisms are interlinked. In adult humans, an EPA plus DHA intake greater than 2 g day^–1 seems to be required to elicit anti‐inflammatory actions, but few dose finding studies have been performed. Animal models demonstrate benefit from n‐3 fatty acids in rheumatoid arthritis (RA), inflammatory bowel disease (IBD) and asthma. Clinical trials of fish oil in patients with RA demonstrate benefit supported by meta‐analyses of the data. Clinical trails of fish oil in patients with IBD and asthma are inconsistent with no overall clear evidence of efficacy.