This volume contains the papers presented at WINE 2009: the 5th International Workshop on Internet and Network Economics held December 14-18, 2009, in Rome, at the Department of Computer and System Sciences, Sapienza University of Rome. Over the past decade, there has been growing interaction between researchers in theoretical computer science, networking and security, economics, mathematics, sociology, and management sciences devoted to the analysis of problems arising in the Internet and the worldwide web. The Workshop on Internet and Network Economics (WINE) is an interdisciplinary forum for the exchange of ideas and results arising in these varied fields. There were 142 submissions to the workshop, including regular and short papers. All submissions were rigorously peer-reviewed and evaluated on the basis of the quality of their contribution, originality, soundness, and significance. Almost all submissions were accepted.