The demographic transition, which began around 1800 with declining mortality in Europe, has spread worldwide and is projected to be completed by 2100. This transition has led to significant changes in population growth rates, fertility, and mortality, reshaping economic and demographic life cycles. Since 1800, global population has increased sixfold, and by 2100, it is expected to rise tenfold. The ratio of elderly to children will increase tenfold, while the length of life will triple and the number of births per woman will drop from six to two. These changes have raised questions about the causes of population growth before the transition, the role of medical progress and income in declining mortality, and the future of life expectancy and fertility. The demographic transition has also led to a shift in age distribution, with a growing proportion of the population becoming elderly. This shift has implications for economic development, pension systems, and social policies, particularly in more developed countries where population aging is more pronounced. The transition is ongoing, and no country has yet completed it, as even industrial nations are projected to age rapidly in the coming decades.The demographic transition, which began around 1800 with declining mortality in Europe, has spread worldwide and is projected to be completed by 2100. This transition has led to significant changes in population growth rates, fertility, and mortality, reshaping economic and demographic life cycles. Since 1800, global population has increased sixfold, and by 2100, it is expected to rise tenfold. The ratio of elderly to children will increase tenfold, while the length of life will triple and the number of births per woman will drop from six to two. These changes have raised questions about the causes of population growth before the transition, the role of medical progress and income in declining mortality, and the future of life expectancy and fertility. The demographic transition has also led to a shift in age distribution, with a growing proportion of the population becoming elderly. This shift has implications for economic development, pension systems, and social policies, particularly in more developed countries where population aging is more pronounced. The transition is ongoing, and no country has yet completed it, as even industrial nations are projected to age rapidly in the coming decades.