People first: human beings are the goal and the actors of sustainable development; this chapter presents a brief analysis of how many people there are and where they live. Looking at the largest countries by population it emerges that half of the world people live in Asia. Another significant indicator is population density, a rough way to describe the pressure of population on natural resources. In many developing countries there is an important phenomenon of urban migration, with a growing number of megacities, a fact with relevant impact on development programs. Population changes through time, but the speed of this change is quite different in the regions of the world; it is necessary to investigate the causes of population dynamics and the challenges it poses. Demographic transition is a process taking place all around the world, but countries are at very different stages and it is important to examine the indicators characterizing it. Infant and child mortality rates and life expectancy at birth are three aspects which directly impact on mortality rates; while fertility is the factor which by and large explains the natality rate. Fertility is complex magnitude, which can be affected by many social, economic, and cultural elements; China’s one-child policy is a very interesting case. The chapter concludes by presenting the description of the population of a country through its age structure, which has important implications on health and educational policies.