This dissertation contributes to our understanding of citizens' attitudes towards European integration, focusing in particular on the causes and consequences of political identities. What generates a collective identity in a political community of different nations? What effects does a collective European identity have on relations among EU-citizens and their approval of authority transfers to supranational institutions? The first study examines the structure of citizens' attitudes towards the EU. Based on an analysis of Eurobarometer data and the Chapel Hill expert survey I show that citizens are more ambivalent and less positive about the EU when elite division on European integration is more pronounced. Affective cues, such as an attachment to the EU, are powerful factors that explain both low levels of ambivalence and low levels of indifference towards the EU. In the second study I test Karl Deutsch's argument on the role of social interactions for community building in Europe. I apply insights from social psychology to explain when and why contact between individuals from different national backgrounds can lead to a collective European identity. I conducted a panel survey with about 1500 students from 38 German universities. Individuals were surveyed before, during, and after their stay in another European country. My results show that contact with other international students rather than contact with students from the host country fosters a collective European identity most effectively. Also, transnational social interactions have a more profound impact on individuals with a weak European identity to begin with. In the third study I use the same panel data to test implications of this identity change. A more pronounced European identity is related to viewing other Europeans as co-citizens with equal rights rather than as immigrants with limited rights. Individuals who develop a stronger European identity are more likely to support an equal access to jobs and local elections for citizens from other EU member states in Germany. Additionally, identity change motivates an endorsement for the transfer of political authority to the EU level. However, identity change affects left leaning citizens' attitudes more directly than attitudes of their right leaning peers.