Aging is a major risk factor for several common neurodegenerative diseases, including Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), and Huntington's disease (HD). Recent studies have implicated mitochondrial dysfunction and oxidative stress in the aging process and also in the pathogenesis of neurodegenerative diseases. In brain and other tissues, aging is associated with progressive impairment of mitochondrial function and increased oxidative damage. In PD, several studies have demonstrated decreased complex I activity, increased oxidative damage, and altered activities of antioxidant defense systems. Some cases of familial ALS are associated with mutations in the gene for Cu, Zn superoxide dismutase (Cu, Zn SOD) and decreased Cu, Zn SOD activity, while in sporadic ALS oxidative damage may be increased. Defects in energy metabolism and increased cortical lactate levels have been detected in HD patients. Studies of AD patients have identified decreased complex IV activity, and some patients with AD and PD have mitochondrial DNA mutations. The age-related onset and progressive course of these neurodegenerative diseases may be due to a cycling process between impaired energy metabolism and oxidative stress.