The notion of global governance is widely studied in academia and increasingly relevant to politics and policy making. Yet many of its fundamental elements remain unclear in both theory and practice. This book offers a fresh perspective by analyzing global governance in terms of three major trends, as exemplified by developments in global sustainability governance: the emergence of nonstate actors; new mechanisms of transnational cooperation; and increasingly segmented and overlapping layers of authority. The book, which is the synthesis of a ten-year "Global Governance Project" carried out by thirteen leading European research institutions, first examines new nonstate actors, focusing on international bureaucracies, global corporations, and transnational networks of scientists; then investigates novel mechanisms of global governance, particularly transnational environmental regimes, public-private partnerships, and market-based arrangements; and, finally, looks at fragmentation of authority, both vertically among supranational, international, national, and subnational layers, and horizontally among different parallel rule-making systems. The implications, potential, and realities of global environmental governance are defining questions for our generation. This book distills key insights from the past and outlines the most important research challenges for the future.