This book reviews research on presently known theories and mechanisms responsible for the rate of aging and maximum longevity of different animals including birds, mammals and humans (to explain "how" do we age and the velocity of the aging process). In addition, this book deals with the historical evolutionary explanations of aging and longevity ("why" do we age). The mechanisms discussed include mitochondria and oxygen free radicals, the presence of macromolecules constitutively highly resistant to deleterious modification in the tissues of long-lived animals, insulin/IGF-1 like signalling, telomere shortening, dietary restrictions, and other proposed molecular mechanisms.