Recent studies of postmortem brains from Alzheimer's disease (AD) patients and transgenic mouse models of AD suggest that oxidative damage, induced by amyloid β (Aβ), is associated with mitochondria early in AD progression. Aβ and amyloid-precursor protein are known to localize to mitochondrial membranes, block the transport of nuclear-encoded mitochondrial proteins to mitochondria, interact with mitochondrial proteins, disrupt the electron-transport chain, increase reactive oxygen species production, cause mitochondrial damage and prevent neurons from functioning normally. Furthermore, accumulation of Aβ at synaptic terminals might contribute to synaptic damage and cognitive decline in patients with AD. Here, we describe recent studies regarding the roles of Aβ and mitochondrial function in AD progression and particularly in synaptic damage and cognitive decline.