This book constitutes the thoroughly refereed post-proceedings of the 4th International Workshop on Agents and Peer-to-Peer Computing, AP2PC 2005, held in Utrecht, Netherlands, July 2005, in the context of the 4th International Joint Conference on Autonomous Agents and Multi-Agent Systems, AAMAS 2005. The 13 revised full papers cover trust and reputation, P2P infrastructure, semantic infrastructure, as well as community and mobile applications.

Peer-to-peer (P2P) computing promises to o?er exciting new possibilities in d- tributed information processing and database technologies. The realization of this promise lies fundamentally in the availability of enhanced services such as structured ways for classifying and registering shared information, veri?cation and certi?cation of information, content-distributed schemes and quality of c- tent, security features, information discovery and accessibility, interoperation and composition of active information services, and ?nally market-based me- anisms to allow cooperative and non-cooperative information exchanges. The P2P paradigm lends itself to constructing large-scale complex, adaptive, - tonomous and heterogeneous database and information systems, endowed with clearly speci?ed and di?erential capabilities to negotiate, bargain, coordinate, and self-organize the information exchanges in large-scale networks. This vision will have a radical impact on the structure of complex organizations (business, scienti?c, or otherwise) and on the emergence and the formation of social c- munities, and on how the information is organized and processed. The P2P information paradigm naturally encompasses static and wireless connectivity, and static and mobile architectures. Wireless connectivity c- bined with the increasingly small and powerful mobile devices and sensors pose new challenges to as well as opportunities for the database community. Inf- mation becomes ubiquitous, highly distributed and accessible anywhere and at any time over highly dynamic, unstable networks with very severe constraints on the information management and processing capabilities.

Peer-to-peer (P2P) computing is currently attracting enormous public attention, spurred by the popularity of file-sharing systems such as Napster, Gnutella, Morpheus, Kaza, and several others. In P2P systems, a very large number of autonomous computing nodes, the peers, rely on each other for services. P2P networks are emerging as a new distributed computing paradigm because of their potential to harness the computing power and the storage capacity of the hosts composing the network, and because they realize a completely open decentralized environment where everybody can join in autonomously. Although researchers working on distributed computing, multiagent systems, databases, and networks have been using similar concepts for a long time, it is only recently that papers motivated by the current P2P paradigm have started appearing in high quality conferences and workshops. In particular, research on agent systems appears to be most relevant because multiagent systems have always been thought of as networks of autonomous peers since their inception. Agents, which can be superimposed on the P2P architecture, embody the description of task environments, decision-support capabilities, social behaviors, trust and reputation, and interaction protocols among peers. The emphasis on decentralization, autonomy, ease, and speed of growth that gives P2P its advantages also leads to significant potential problems. Most prominent among these are coordination the ability of an agent to make decisions on its own actions in the context of activities of other agents, and scalability the value of the P2P systems in how well they self-organize so as to scale along several dimensions, including complexity, heterogeneity of peers, robustness, traffic redistribution, etc.

This book brings together an introduction, three invited articles, and revised versions of the papers presented at the Second International Workshop on Agents and Peer-to-Peer Computing, AP2PC 2003, held in Melbourne, Australia, July 2003."