Three main challenges are addressed here, with a special focus on emerging countries: one is the environmental impact of still growing populations in low-income countries on the use of the territory and its resources. In many of them, especially the poorest ones, this is a driving force behind deforestation and degradation of ecosystems, since a large fraction of their people depend on nature for their livelihood. So, there is positive feedback from social and economic progress to the protection of nature and reduction of greenhouse gases emissions. However, lower population growth also means a slower expansion of the labor force, and this could translate into slower economic progress if not compensated by higher productivity growth, something hard to achieve in less advanced countries. Finally, the financial impacts of aging on the social security systems are also addressed. This issue is reaching critical proportions in many advanced countries, where this process started earlier, and also in several large and medium size emerging countries where the demographic transition has proceeded very fast in the last few decades.