Polychaetes are segmented worms belonging to the phylum Annelida. They occur in a wide range of habitats and dominate in several marine and estuarine benthic communities, where they play an important role in the ecosystem. Polychaetes are an ancient invertebrate group dating back to the Middle Cambrian and possibly earlier. For decades, considerable research has been undertaken on the group. To coordinate the apparently disparate interests of polychaete researchers throughout the world, the International Polychaete Association was created in 1986 during the the second of a series of International Polychaete Conferences that have been held at three year intervals ever since. This volume consists of papers selected from the communications presented at the Eighth International Polychaete Conference, held in July 2004, at the Universidad Autónoma de Madrid (UAM), Spain. The Conference was organized by the UAM and the Consejo Superior de Investigaciones Científicas (CSIC). The published papers cover a wide range of topics related to the study of polychaetes, from taxonomy and systematics, to ecology, biogeography, and morphology, including reproduction, development and other miscellaneous topics. As a whole, these papers perfectly illustrate the main topics that are currently driving scientific advances in polychaete research.