Current users typically interact with the Internet through the use of a Web browser and a client/server based connection to a Web server. However, as we move forward to allow true machine-to-machine communication, we are in need of more scalable solutions which employ the use of decentralized te- niques to add redundancy, fault tolerance and scalability to distributed s- tems. Distributed systems take many forms, appear in many areas and range from truly decentralized systems, like Gnutella and Jxta, centrally indexed brokered systems like Web services and Jini and centrally coordinated s- tems like SETI@Home. From P2P to Web Services and Grids: Peers in a client/server world p- vides a comprehensive overview of the emerging trends in peer-to-peer (P2P), distributed objects, Web services and Grid computing technologies, which have rede?ned the way we think about distributed computing and the Int- net. This book has two main themes: applications and middleware. Within the context of applications, examples of the many diverse architectures are provided including: decentralized systems like Gnutella and Freenet; brokered ones like Napster; and centralized applications like SETI and conventional Web servers. For middleware, the book covers Jxta, as a programming - frastructure for P2P computing, along with Web services, Grid computing paradigms, e. g. , Globus and OGSA,and distributed-object architectures, e. g. , Jini. Each technology is described in detail, including source code where - propriate, and their capabilities are analysed in the context of the degree of centralization or decentralization they employ.