Global population growth is generally considered to be one of the major driving forces of global change. Population changes, however, follow their own dynamics and can be influenced only marginally in the short run. This paper outlines the basic structure of these dynamics, including the important momentum of population growth. It also presents alternative population projections that show that in the long run much depends on fertility trends in the near future, and that a successful curbing of population growth will result in very rapid and significant population aging.