Alzheimer disease is a progressive dementia with unknown etiology that affects a growing number of the aging population. Increased expression of inflammatory mediators in postmortem brains of people with Alzheimer disease has been reported, and epidemiological studies link the use of anti-inflammatory drugs with reduced risk for the disorder. On the initial basis of this kind of evidence, inflammation has been proposed as a possible cause or driving force of Alzheimer disease. If true, this could have important implications for the development of new treatments. Alternatively, inflammation could simply be a byproduct of the disease process and may not substantially alter its course. Or components of the inflammatory response might even be beneficial and slow the disease. To address these possibilities, we need to determine whether inflammation in Alzheimer disease is an early event, whether it is genetically linked with the disease and whether manipulation of inflammatory pathways changes the course of the pathology. Although there is still little evidence that inflammation triggers or promotes Alzheimer disease, increasing evidence from mouse models suggests that certain inflammatory mediators are potent drivers of the disease. Related factors, on the other hand, elicit beneficial responses and can reduce disease.