This book constitutes the proceedings of the 6th International Symposium on Algorithmic Game Theory, SAGT 2013, held in Aachen, Germany, in October 2013. The 25 papers presented in this volume were carefully reviewed and selected from 65 submissions. They cover various important aspects of algorithmic game theory, such as solution concepts in game theory, efficiency of equilibria and the price of anarchy, computational aspects of equilibria and game theoretical measures, repeated games and convergence of dynamics, evolution and learning in games, coordination and collective action, network games and graph-theoretic aspects of social networks, voting and social choice, as well as algorithmic mechanism design.