The economic crisis is also a crisis for economic theory. Most analyses of the evolution of the crisis invoke three themes, contagion, networks and trust, yet none of these play a major role in standard macroeconomic models. What is needed is a theory in which these aspects are central. The...

1. Introduction -- 2. The structure of interaction -- 3. Fish markets : an example of the emergence of aggregate coordination -- 4. Financial markets: bubbles, herds and crashes -- 5. Public goods : a coordination problem -- 6. Segregation : Schelling's model -- 7. Conclusion.