Mortality is a demographic component that contributes to shaping the size, structure, and dynamics of populations. Life expectancy has been rising remarkably in the more developed countries since the 19th century and the process of rising life expectancy also has begun in most of the less developed countries. Increases in adult life expectancy and declines in birth rates result in aging societies. Survival is increasing as a result of progress in economic development, social improvements, and advances in medicine. However, death rates vary significantly in different parts of the world and are particularly high in sub-Saharan Africa.