Global environmental politics has been determined by a strong increase of global governance systems in the past few decades. This study draws a causal connection between the activities of non-state actors and legitimacy. Only if non-state actors can bring their political and problem-solving potential to bear, it will be possible to improve the legitimacy of global governance systems.In this book, theoretical and empirical chapters balance one another. Four theoretical and conceptual chapters illustrate the relevance and meaning of legitimacy, the impact of non-state actors on environmental governance, and describe various methodological issues involved with the coding of 23 environmental regimes. The five empirical chapters explore whether the prime hypothesis can be verified.