Peer-to-peer systems are already being used for a vast number of applications in content exchange, but mostly searching is done by simple keyword lookups. In contrast, information Retrieval techniques vastly differ from retrieval in pure data management systems. Information retrieval means that not only some more or less matching objects have to be retrieved, but a list of the best matching objects over the entire network given a user's information needs. Since the 1960ies the information retrieval community considers ways to efficiently and effectively query document collections and designs dedicated retrieval systems like e.g. SMART. Since the content offered in P2P networks currently evolves from simple files that can be adequately described by some meta-data tags, to collections of complex documents that need somewhat more detailed descriptions, the development of refined retrieval techniques has to be fostered.