Reactive oxygen species are important for many life sustaining processes of cells and tissues, but they can also induce cell damage and death. If their production and levels within cells are not effectively controlled, then the detrimental effects of oxidative stress can accumulate. Oxidative stress is widely thought to underpin many ageing processes, and the oxidative stress theory of ageing is one of the most widely acknowledged theories of ageing. As well as being the major source of reactive oxygen species, mitochondria are also a major site of oxidative damage. The purpose of this review is a concise and current review of the role of oxidative stress in ageing process. Emphasis is placed upon the roles of mitochondrial proton leak, the uncoupling proteins, and the anti-ageing effects of caloric restriction.