Normal human fibroblasts cultured in vitro only have a limited proliferation potential. They eventually become senescent as a result of serial passage, which is commonly known as replicative senescence. This led to the suggestion that cellular senescence might be a cellular basis of human aging. Indeed, cells with the characteristics of senescence accumulate with age in multiple tissues from both humans and rodents, thus implying a role of cellular senescence in aging. Cellular senescence in vitro has, therefore, been regarded as a useful model for elucidating molecular mechanisms that underlie organismal aging. In addition to replicative senescence, cellular senescence can also be induced by various stresses including oxidative stress. Hydrogen peroxide is widely used to achieve oxidative stress-induced premature senescence within a short period of time. Such induced premature senescent cells display many markers that are indistinguishable from replicative senescent cells. Thus, oxidative stress-induced senescent cells serve as an excellent in vitro tool for aging research.