Game Theory Online is a joint project by Matt Jackson (Stanford University), Kevin Leyton-Brown (University of British Columbia) and Yoav Shoham (Stanford University) to bring free, high-quality information about game theory to the world. Popularized by movies such as "A Beautiful Mind", game theory is the mathematical modeling of strategic interaction among rational (and irrational) agents. Beyond what we call 'games' in common language, such as chess, poker, soccer, etc., it includes the modeling of conflict among nations, political campaigns, competition among firms, and trading behavior in markets such as the NYSE. How could you begin to model eBay, Google keyword auctions, and peer to peer file-sharing networks, without accounting for the incentives of the people using them?