“Demography is destiny” is an oft-cited phrase that suggests the size, growth, and structure of a nation’s population determines its long-term social, economic, and political fabric. The phrase highlights the role of demographics in shaping many complex challenges and opportunities societies face, including several pertinent to economic growth and development. Nevertheless, it is an overstatement to say that demography determines all, as it downplays the fact that both demographic trajectories and their development implications are responsive to economic incentives; to policy and institutional reforms; and to changes in technology, cultural norms, and behavior. The world is undergoing a major demographic upheaval with three key components: population growth, changes in fertility and mortality, and associated changes in population age structure.