This book constitutes the refereed proceedings of the 5th International Symposium on Algorithmic Game Theory, SAGT 2012, held in Barcelona, Spain, in October 2012. The papers present original research at the intersection of algorithms and game theory and address various current topics such as solution concepts in game theory, efficiency of equilibria and price of anarchy, complexity classes in game theory, computational aspects of equilibria, computational aspects of fixed-point theorems, repeated games, evolution and learning in games, convergence of dynamics, coalitions, coordination and collective action; reputation, recommendation and trust systems, graph-theoretic aspects of social networks, network games; cost-sharing algorithms and analysis, computing with incentives, algorithmic mechanism design, computational social choice, decision theory, and pricing, auction algorithms and analysis, economic aspects of distributed computing, internet economics and computational advertising.