Docosahexaenoic acid (DHA, C22:6, ω-3) is a highly polyunsaturated omega-3 fatty acid. It is concentrated in neuronal brain membranes, for which reason it is also referred to as a "brain food". DHA is essential for brain development and function. It plays an important role in improving antioxidant and cognitive activities of the brain. DHA deficiency occurs during aging and dementia, impairs memory and learning, and promotes age-related neurodegenerative diseases, including Alzheimer's disease (AD). For about two decades, we have reported that oral administration of DHA increases spatial memory acquisition, stimulates neurogenesis, and protects against and reverses memory impairment in amyloid β peptide-infused AD rat models by decreasing amyloidogenesis and protects against age-related cognitive decline in the elderly. These results demonstrate a robust link between DHA and cognitive health. Rodents that were fed a diet low in ω-3 polyunsaturated fatty acids, particularly those that were DHA-deficient, frequently suffered from anxiety, depression and memory impairment. Although the exact mechanisms of action of DHA in brain functions are still elusive, a host of mechanisms have been proposed. For example, DHA, which inherently has a characteristic three-dimensional structure, increases membrane fluidity, strengthens antioxidant activity and enhances the expression of several proteins that act as substrates for improving memory functions. It reduces the brain amyloid burden and inhibits in vitro fibrillation and amyloid-induced neurotoxicity in cell-culture model. In this review, we discuss how DHA acts as a molecule with diverse functions.