Alzheimer's disease (AD) is characterized by an accumulation in the brain of amyloid beta peptides (Abeta). The production of Abeta requires two sequential cleavages induced by beta- and gamma-secretases on the beta-amyloid precursor protein (APP). Altered activity of these secretases is involved in the pathogenesis of AD. The expression and activity of beta-secretase (BACE1) is augmented in the brain in late-onset sporadic AD. Mutant presenilin 1 (PS1), the major genetic defect of early-onset familial AD (FAD), alters the activity of gamma-secretase, leading to increased production of Abeta42. Here we review the role of oxidative stress as a molecular link between the beta- and the gamma-secretase activities, and provide a mechanistic explanation of the pathogenesis of sporadic late-onset AD. We also discuss evidence for a role of the same mechanism in the pathogenesis of familial AD carrying PS1 mutations.