The combination of a study on ECEC quality with a behavioral genetic design of twin families provides additional opportunities to investigate the impact of childcare quality for both methodological and substantive reasons. Methodologically, the twin family design is a strong tool to control for unobserved heterogeneity in the relationships between child development, family resources and processes, and childcare institutions. Thus, the disentangling of genetic and social origin allows for a more accurate inquiry of how ECEC institutions interact with the family of origin to produce child outcomes in the short and in the long run. In substantive terms, we assess how much of the within and between-family variation in twins‘ development at age five is accounted for by their ECEC experience including quality aspects of the ECEC institution they attend. We will examine if positive social environments, such as high quality ECEC institutions, can compensate for lacking family resources and buffer family stress by providing specific forms of social support or a positive sense of social control (e.g. Shanahan et al. 2008). Moreover, we investigate whether the same ECEC experience leads to different developmental outcomes dependent on children’s genetic endowments. In other words, do children suffer or profit differently from the same ECEC characteristics? In these respects, the K²iD-twins sample serves to extend the methodological and theoretical approach of the SOEP-ECEC Quality Project.