The diversification in family practices constitutes a major theme of research in the field of population studies. A substantial part of these diversification processes are linked to the stage of family formation. The increase in non-marital cohabitation and non-marital fertility and the postponement of marriage and parenthood have particularly characterised the transition from youth to adulthood among the post-war cohorts. In studying the transition from youth to adulthood, the timing of events is crucial. Different timing patterns are the product of the interplay between a multitude of factors in the institutional as well as the personal context. In this respect, cross-country comparative research is a powerful tool to understand the determinants of the change processes. To move beyond the mere description of the changes is a challenge that not only is eminently scientific, it also gives the necessary insights to grasp the implications of the changes for policy. From this policy perspective, the postponement of the integration into adulthood has major societal consequences, such as the very low fertility and the fact that the family formation itself became a matter of choice. Over the years, the Population and Family Studies Centre (CBGS), a scientific institute of the Flemish Community, has intensively investigated these changes constituting the Second Demographic Transition, not only through national but through cross-national comparative research.