In an effort to combine research in peer-to-peer systems with techniques for incremental backup systems, we propose pStore: a secure distributed backup system based on an adaptive peer-to-peer network. pStore exploits unused personal hard drive space attached to the Internet to provide the distributed redundancy needed for reliable and effective data backup. Experiments on a 30 node network show that 95 % of the files in a 13 MB dataset can be retrieved even when 7 of the nodes have failed. On top of this reliability, pStore includes support for file encryption, replication, versioning, and sharing. Its custom versioning system permits arbitrary version retrieval similar to CVS. pStore provides this functionality at less than 10 % of the network bandwidth and requires 85 % less storage capacity than simpler local tape backup schemes for a representative workload.