After the “demographic explosion” that took place in the last two centuries, the pressure of population growth became a driving force of environmental degradation at the global level. This is the first of two chapters dealing with this issue. In this one the general trends of the global demographic transition are described and explained, with a special focus on the interactions between population dynamics and economic development. The well stablished relationship between economic and social progress and the reduction of fertility rates is a critical one to explain the current evolution of population across the world. The positive feedback from human progress into lower fertility is one key reason to promote investment in human capital—specially among young women—in the poorest regions of the world as a pre-condition for sustainable development at the global level. The other key demographic issue addressed here is the challenge posed by aging populations to those countries that are farther ahead in their demographic transition, including now several large and small emerging countries.