This chapter argues that population dynamics have long been inextricably linked to ideas about social sustainability. It contends that our ability to achieve sustainable development may well depend on the dynamics of the world's population. The chapter then looks at the rising global human population, and examines how to cope with world population dynamics as part of the social sustainability challenge. It considers changing population dynamics for each of the world regions. However, what may ultimately matter most for social sustainability and social wellbeing is how equitable a country is. When a country is equitable, like Finland, people do not have to think of having more children in future to help pay for their own old age. It may seem impossible to think that this will ever happen to some of the poorest and most inequitable countries but, as the chapter points out, just a century ago Finland was a relatively poor country.