Recently, there has been an increase in research into the relationship between long-chain n-3 polyunsaturated fatty acids (n-3 PUFAs) and cognitive function although the topic remains controversial. In epidemiological studies, fatty fish or n-3 PUFAs consumption has been found to be associated with a reduced risk of cognitive decline or dementia. In addition, higher proportion of total n-3 PUFAs or higher ratio of n-3 to n-6 PUFAs on erythrocyte membranes have been found to be associated with a lower risk of cognitive decline. Furthermore, in some animal models, docosahexaenoic acid (DHA, C22:6n-3) or eicosapentaenoic acid (EPA, C22:5n-3) administration have been found to improve learning ability and reduce Aβ amyloid deposition. However, findings from clinical trials of n-3 PUFAs supplementation to improve cognition in older people have been inconsistent. Some sub-group analyses have suggested that people with mild dementia or mild cognitive impairment may benefit most. Contradictory results may be clarified by better controlling possible confounders, more consistency in interventions and outcome measurements, and longer follow-up. Larger but possibly more focused trials should be considered along with efforts to develop better biomarkers for intervention response.