Why do we age, and how do we age ? There have been more than hundreds of theories of aging that attempt rational explanation for these challenging questions. From an evolu- tionary viewpoint, genetic trade-off between longevity and fecundity could explain the evo- lution of aging. Recent applications of new experimental methods and techniques to aging research provided mechanistic insights into the role of oxidative stress, mitochondrial dys- function, cellular senescence, and modulation of the insulin/insulin-like growth factor sig- naling and NAD+-dependent deacetylase sirtuins in aging process. In this article, a long history of aging research is looked back on, and the concepts and theories of aging are summarized.