In a handful of years only, Peer-to-Peer (P2P) systems have become an integral part of the Internet. After a few key successes related to music-sharing (e.g., Napster or Gnutella), they rapidly developed and are nowadays firmly established in various contexts, ranging from large-scale content distribution (BitTorrent) to Internet telephony(Skype) or networking platforms (JXTA). The main idea behind P2P is to leverage on the power of end-computers: Instead of relying on central components (e.g., servers), services are powered by \emph{decentralized overlay} architectures where end-computers connect to each other dynamically. So far, much effort has been devoted to the development of distributed hashtables (DHTs) to index data in a totally decentralized way. Though extremely robust and scalable, these systems suffer from simplistic data models, which mainly consist of unstructured collections of key-value pairs. More recently however, a significant number of uncorrelated research efforts focused on enriching P2P systems with more expressive data models and query languages. As a result, various Semantic Overlay Networks (SON) supporting relational, semi-structured or triple-based collections of data over decentralized P2P networks started to appear. SON can be considered as truly unique as they aim at managing structured data in very large-scale, decentralized, heterogeneous and highly dynamic environments.