Cholesterol, omega-3 fatty acids, and triglycerides have been postulated to play roles in affecting cognition in Alzheimer's disease (AD), the elderly, and obesity. Animal, human epidemiological, and in vitro studies each suggest an important role for cholesterol in the regulation of amyloid-beta (Abeta) protein and the pathogenesis of AD. In contrast, well controlled studies have failed to show an effect of cholesterol lowering with statins on cognition, indicating that the cholesterol effect is spurious or indirect, possibly mediated through other lipids. Administration of diedocosahexanoic acid (DHA), a dietary omega-3 fatty acid derived primarily from fish and plants, improves cognition and reduces lipid peroxidation in animals, including in mouse models of AD. DHA also blocks Abeta-mediated tau phosphorylation. In humans, fish consumption or administration of DHA has been associated with cognitive improvement in many, but not all, studies. Both human and animal studies show that obesity is associated with cognitive impairments and that lowering triglycerides improves cognition. Administration of triglycerides to mice decreases learning and memory and impairs long-term potential. The effect of triglycerides may be mediated in part by inducing resistance to positive cognitive features of gastrointestinal hormones such as leptin. Overall, these studies strongly suggest that some lipids affect cognition in AD, the elderly, and obesity through a variety of mechanisms yet to be fully defined.