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