Peer-to-peer (P2P) Web search has gained a lot of interest lately, due to the salient characteristics of P2P systems, namely scalability, fault-tolerance and load-balancing. However, the lack of global knowledge in a vast and dynamically evolving environment like the Web presents a grand challenge for organizing content and providing efficient searching. Semantic overlay networks (SONs) have been proposed as an approach to reduce cost and increase quality of results, and in this paper we present an unsupervised approach for distributed and decentralized SON construction, aiming to support efficient search mechanisms in unstructured P2P systems. Categories and Subject Descriptors H.3.3 [Information storage and retrieval]: Information search and retrieval; H.3.1 [Information storage and retrieval]: Content Analysis and Indexing; I.7.m [Document and text processing]: Miscellaneous General Terms Algorithms, Design, Performance Keywords Distributed and peer-to-peer search, semantic overlay...