Attempts to define ageing, explain theories and classify them have always been an important issue for biogerontologists. Oxidative stress, telomeres, genetics, hormonal changes, immunity and damage accumulation over threshold values are all common theories that have been studied and modified over a long period of time. Verifications of these theories may lead to enlightenment about molecular mechanisms, and these can give rise to new research to reverse or slow age related pathological changes and increase average life span. Nowadays, an increased ratio of "successful ageing" or "healthier ageing" is a big aim for the whole society. Achieving this purpose also depends on research which studies molecular mechanisms and routine laboratory markers of ageing.