The MapIE project aims to identify mechanisms causing educational inequalities and policies or interventions compensating for them. We do a systematic review of how these have been studied internationally using longitudinal designs. We map existing longitudinal data that we describe in a public metadata database, and we analyse them to describe the mechanisms of inequalities and to identify effective policies and interventions in different local contexts. We focus on the contextual differences between the Nordic and Central European countries that have structurally different education systems. Within national contexts and using comparative research designs, we look at system-, regional- and school-level policies and practices and individual-level factors (e.g. socioeconomic status, immigrant background, gender, special educational needs) that are associated with the development of educational inequalities and the successful prevention of achievement gaps. We publish and disseminate the empirical results of the project through multiple channels to scientific audiences, policy makers and the wider public. At the end, we create a public framework for studying educational inequalities longitudinally in future studies, acknowledging the role of local contexts but aiming for comparability at the European level. We implement the project in seven work packages. Our consortium consists of internationally acknowledged experts in large-scale educational assessment research from Finland, Sweden, Norway, Hungary and Luxembourg/Germany. The partners have a long-standing history of collaboration with each other, European researchers, and national and international education policy makers, and they bring extensive longitudinal datasets to the project. The project will considerably strengthen the knowledge base of what shapes educational outcomes over time, how to best support the learning of all students and how to reduce education inequalities in the short, medium and long term.