How can we conceptualize and empirically measure political trust and legitimacy beyond the usual survey question “How much trust do you have in the parliament?”? Does the multi-level nature of European representative democracies require an identical level of citizen support at the regional, national and EU levels? How does social polarization on key policy issues of our times –immigration, climate change, and gender inequality– challenge the political trust in, and legitimacy of, democratic political systems? And what can policymakers and civil society do to master these challenges? ActEU aims at finding answers to these questions pursuing two overarching goals: In phase 1, we map and investigate persistent problems of declining trust, legitimacy and representation in Europe with a particular attention to the polarization of societies and the EU’s multi-level structures. Providing an innovative conceptual framework on political attitudes, behavior and representation across Europe, we establish an original empirical infrastructure based on an innovative combination of methods and newly collected quantitative and qualitative empirical data (focus groups, experimental surveys, web scraping). In phase 2, these results will flow directly into the creation of a toolbox of remedial actions to enhance political trust in and legitimacy of European representative democracies. In cooperation with a newly created Civil Society Network, Youth Democracy Labs across 13 European cities and in exchange with political cartoonists “Cartooning for democracy”, we will develop context-sensitive solutions for all polity levels and some of the most polarizing policy areas, and craft tailor-made toolkits for both policymakers and civil society and the educational sector. Finally, we deploy a differentiated dissemination strategy to maximize ActEU’s scientific, policy and societal impact in activating European citizens’ trust and working towards a new era of representative democracy.