This monograph reviews the epidemiological, demographic, and biological basis of population models of human mortality. These investigations were motivated by the desire to better understand the regularities of survival processes among adults -- especially at extreme late ages where empirical data is currently limited. The monograph discusses biological mechanisms, which shape the age-patterns of mortality. The effects of an individual health state, susceptibility to diseases and death, or physical frailty on changes in late age survival are also investigated.