Based on the NEPS data it will be possible to analyze educational trajectories and their determinants over time across five different starting cohorts. Adopting an interdisciplinary perspective and based on analyses of NEPS data, the project aims at clarifying the role of immigrants’ language proficiency for their social integration in the context of their country of residence. Emanating from a resource approach, language proficiency is seen as a central resource for social integration, especially structural integration into the educational system. While it is generally accepted that the language of the country of residence (L2) plays a crucial role for integration, the effects of the language of origin (L1) are highly controversial. The main goal of the research lies in illuminating the role of L1 and L2 for educational success. More specifically, the impact of language proficiency for different aspects of competence development, educational decisions and eventual school success will be examined. In addition, immigrants’ language proficiency will be studied in relation to other aspects of social integration, such as labor market outcomes, psychological well-being and social adaptation as well as cultural identity. In exploring determinants of proficiency development in L1 and L2 at different levels, moreover, the prevalence and the effects of language support in L1 and L2 will be analyzed. To answer an important methodological question that is relevant for research on immigrant integration more generally, finally, there are plans to examine whether self-reports of proficiency in L1 and L2 yield the same results as objective measurements.