Understanding the impact of regime complexes on global governance calls for creative policy thinking. This introduction provides a new and more precise definition of the concept of regime complex. It also suggests spe cific tools to characterize regime complexes and analyze their impacts on global governance. The articles in this issue deepen the analytical under standing of complexes by examining concrete examples in various domains of global governance such as piracy, taxation, energy, food security, emis sions reduction, carbon sinks, biosafety, and refugee governance. In addi tion to providing an in-depth description of a variety of different regime complexes, this issue is innovative on three accounts: (1) it presents com plexes as both barriers and opportunities for global governance and gives explanations for these diverse outcomes; (2) it shows how a broad spec trum of actors is necessary for understanding the creation and evolution of complexes; and (3) it qualifies former claims to the effect that only pow erful actors can impact regime complexes. KEYWORDS : regime complexes, networks, institutional centralization, institutional fragmentation, institu tional density. A GROUP OR A SYSTEM HAS PROPERTIES THAT DIFFER FROM THOSE OF ITS constitutive parts. Galaxies do not rotate at the same speed as stars; ecosys tems evolve in a more stable manner than their biological components; crowds are usually more impulsive than individual human beings; and H 2 O quenches thirst much better than two spoonfuls of hydrogen and one of oxygen. There is nothing new about this observation. Scientists have known for a long time how important it is to distinguish systems from their com ponents. Yet scholars of international relations often fail to seriously con sider systems populated with international institutions. In earlier times, most intergovernmental organizations (IGOs) and multilateral treaties were relatively independent from one another. But as the number of new treaties has grown at an exponential rate and existing intergovernmental organizations have crept into neighboring issue areas, global governance has become denser. It is no longer possible to negotiate new arrangements on a clear institutional table. One of the most recent multilateral environmental agreements, the 2010 Nagoya Protocol on Access to