This book examines the integration of major trade unions from the six biggest countries of EU's Eastern enlargement into EU governance structures. Based on extensive empirical research, including more than 150 in-depth interviews, statistical data collection, document research, and eight detailed case studies, the contributions describe the activities and perceptions of the trade unions under investigation and the different levels of engagement, including European umbrella organizations, interregional cooperation, and European Works Councils. The book thus contributes to political science research on interest representation and Europeanization as well as sociological research on labor relations.