This Green Book covers a wide spectrum of topics, bringing together perspectives from leading experts in various scientific disciplines, ranging from demography, sociology, psychology and epidemiology, to economics, politics and history. In their short contributions, the authors present evidence that sheds light on these fundamental questions. It is clear that the topics discussed here do not provide a complete picture of the diversity of demographic change, which includes an even wider range of questions, for example, related to family and migration policies.