European social movement organizations (ESMOs) are often considered as ideal mediators between the European political decision-making and the European citizens. In this way, these organizations can possibly contribute to the legitimization of European policy decisions. Critics do however argue that ESMOs often have a "too elitist" character and they, therefore, question the extent to which ESMOs can indeed fulfill this bridging role. This dissertation explores how ESMO legitimacy can be studied. By distinguishing between different aspects of legitimacy and by looking at two particular case studies - the trade union movement and the environmental movement - the book offers insight into how the legitimacy of ESMOs can be understood in the specific multi-level context in which they are working. It offers both a better theoretical insight into the concept of "legitimacy" and an organizational perspective of the internal dynamics within ESMOs and between ESMOs and their national members. The book will be of interest to students and academics in European studies. The in-depth case studies will be of special interest to an NGO/social movement public, and will offer them tools and best practices to work in multi-level interest campaigns. Dissertation.