Annelida Polychaeta III is a new update on the Annelida Polychaeta knowledge on the coast of the Iberian Peninsula and Balearic Islands. This volume includes a detailed study of families Lacydoniidae, Sphaerodoridae, Amphinomidae, Euphrosinidae, Spintheridae, Orbiniidae, Paraonidae, Cossuridae, Nerillidae Opheliidae and consisting of a literature review and in-depth taxonomic each of them as embodied in the development of a general introduction to the family, which summarizes the most important aspects of their anatomy, taxonomy and systematics, as well as the construction of identification keys to genera and species.