Technical Abstract: Evidence that oxidative stress may play a significant role in the development of Alzheimer disease led the authors to investigate the causes and consequences of oxidative stress, which led to a number of surprising findings concerning disease pathogenesis. Evidence suggests that the lesions that are observed in Alzheimer disease, rather than mediating pathogenesis, may instead function as a primary line of antioxidant defense in both the aged and diseased brain. The idea that amyloid beta and tau serve a protective function brings into serious question the rationale of current therapies which are targeted toward removal of the lesions.