Ageing is a universal, intrinsic, progressive and deleterious process. Understanding it is of major interest to scientist, physicians as well as to the general population. Critical to this understanding is to formulate comprehensive theories of aging with high predictive and explanatory power. More than 300 theories have been postulated and are reviewed here. The free radical theory of ageing is one of the most prominent and well studied. It was further developed by one of us (JM) in what has become known as the mitochondrial theory of ageing. These theories provide new experimental approaches to further develop our understanding of the phenomenon of ageing.