This volume offers a synthesis of what is known about very large and very small common-pool resources. Individuals using commons at the global or local level may find themselves in a similar situation. At an international level, states cannot appeal to authoritative hierarchies to enforce agreements they make to cooperate with one another. In some small-scale settings, participants may be just as helpless in calling on distant public officials to monitor and enforce their agreements. Scholars have independently discovered self-organizing regimes which rely on implicit or explicit principles, norms, rules and procedures rather than the command and control of a central authority. The contributors discuss the possibilities and dangers of scaling up and scaling down. They explore the impact of the number of actors and the degree of heterogeneity among actors on the likelihood of cooperative behaviour.