The demographic transition, spanning three centuries, has transformed global population dynamics, moving from high fertility and mortality to low fertility and long life. Beginning around 1800 in Europe, the transition has spread globally, with population expected to increase tenfold by 2100. This shift has led to a tenfold increase in the elderly population relative to children, and a tripling of life expectancy. Women now spend only 14% of their adult years raising children, down from 70% in 1800. These changes raise questions about the causes of mortality and fertility declines, the limits of life expectancy, and the economic and social impacts of an aging population. Mortality decline, driven by medical progress and improved living conditions, preceded fertility decline, which was influenced by economic factors and family planning. The transition has led to significant changes in age distribution, with a growing proportion of elderly and a shrinking proportion of children. In the U.S., life expectancy has more than doubled, and the sex gap in life expectancy has increased. The demographic transition has also affected population growth, with fertility declining in many countries, leading to slower growth and even population decline in some regions. The transition has had profound effects on family structures, health, and economic systems. With fewer children, parents can invest more in each child, and longevity has increased the number of adult years available for other activities. However, aging populations pose challenges, including increased pressure on pension systems and healthcare. In More Developed Countries, the ratio of retirees to workers is rising, prompting policy reforms to address these challenges. The demographic transition is expected to continue, with aging populations in all countries. While some countries have already begun to age, none have completed the transition. The global population is projected to grow significantly, with the most populous countries experiencing the greatest increases. The transition has reshaped economic and social structures, and its long-term implications for human welfare and economic development remain a subject of ongoing debate.The demographic transition, spanning three centuries, has transformed global population dynamics, moving from high fertility and mortality to low fertility and long life. Beginning around 1800 in Europe, the transition has spread globally, with population expected to increase tenfold by 2100. This shift has led to a tenfold increase in the elderly population relative to children, and a tripling of life expectancy. Women now spend only 14% of their adult years raising children, down from 70% in 1800. These changes raise questions about the causes of mortality and fertility declines, the limits of life expectancy, and the economic and social impacts of an aging population. Mortality decline, driven by medical progress and improved living conditions, preceded fertility decline, which was influenced by economic factors and family planning. The transition has led to significant changes in age distribution, with a growing proportion of elderly and a shrinking proportion of children. In the U.S., life expectancy has more than doubled, and the sex gap in life expectancy has increased. The demographic transition has also affected population growth, with fertility declining in many countries, leading to slower growth and even population decline in some regions. The transition has had profound effects on family structures, health, and economic systems. With fewer children, parents can invest more in each child, and longevity has increased the number of adult years available for other activities. However, aging populations pose challenges, including increased pressure on pension systems and healthcare. In More Developed Countries, the ratio of retirees to workers is rising, prompting policy reforms to address these challenges. The demographic transition is expected to continue, with aging populations in all countries. While some countries have already begun to age, none have completed the transition. The global population is projected to grow significantly, with the most populous countries experiencing the greatest increases. The transition has reshaped economic and social structures, and its long-term implications for human welfare and economic development remain a subject of ongoing debate.