On the threshold of a new century the organisers of the European Population Conference 1999 (EPC99) in The Hague decided not only to highlight the population trends that Europe is facing today but also the changes in the 21 st century that are set to shape the future of Europe. They decided to focus on comparative issues, both in time and in space. In order to trace the degree of homogeneity and heterogeneity of European populations over time, converging and diverging population trends that are specific to contemporary and future Europe need to be explored. This is reflected in EPC99's motto: 'European Populations on the threshold of the new millennium; unity in diversity. ' Future demographic developments will be caused by specific economic, social and cultural conditions in Europe, and will, in turn, have a major influence on future economic and social conditions. To the extent that demographic trends differ across countries, separate countries may face different social and economic problems. As demographic trends tend to have long-lasting effects, it is important to assess the possible consequences of future demographic developments at an early stage. On the occasion of EPC99 , two of the organisers, Statistics Netherlands (SN) and the Netherlands Interdisciplinary Demographic Institute (NIDI), decided to produce a set of two population scenarios, showing the possible impact of converging and diverging population trends in the next century.