Due to rapid information growth, peer-to-peer (P2P) systems have become a promising alternative to centralized, client/server-based approaches for large-scale data sharing. By allowing peers to join and leave the system freely, they offer the peers autonomy to control their own resources, high scalability and high robustness against single-peer failures. P2P systems have many applications. This paper focuses on peer-to-peer information retrieval (P2PIR), which aims to retrieve textual documents based on their contents and ranks them based on some relevance measures against the query. The “open nature” of P2P systems and their lack of centralized control pose difficult challenges to the search capability and performance of P2PIR systems. In this paper, we study the recent works on P2P information retrieval, introduce our current research, discuss a few open problems that have not been studied in great depth and propose some future directions for information retrieval research in a...