A comprehensive and exhaustive taxonomic study has been conducted on a significant sample of 13,530 echinoid specimens from the Eocene of Spain. The specimens studied, originating from 350 sites across 146 localities, are housed in eight museums. These sites are distributed across various regions with marine Eocene outcrops, located both on the Iberian Peninsula and in the Balearic Archipelago. As a result of this thorough review, 117 species belonging to 58 genera have been identified, described, and illustrated. The presentation of the data follows modern principles of nomenclature, taxonomy, and systematics for the group, derived from the application of systematic methodologies integrating cladistic analyses with biomolecular and morphological data. It is important to highlight that, until now, no study of this magnitude had been conducted in our country for this group and geological epoch, the Eocene. To achieve accurate chronostratigraphic classification of the materials, contemporary stratigraphic works have been consulted, and numerous field trips have been undertaken to confirm or rectify the stratigraphic positions of the materials. Among the significant contributions of this study is the discovery of new taxon records for Spain and numerous extensions of paleobiogeographic distributions. Moreover, the examination of materials in an optimal state of preservation has allowed for the improvement of descriptions of species whose morphological features were previously known in a limited manner. Additionally, gaps in the knowledge of some insufficiently characterized genera have been identified, suggesting future lines of research or revision works.A notable element of the research has been the identification of species that have served as stratigraphic indicators, thanks to their restricted vertical distribution and broad paleogeographic presence in the European and circum-Mediterranean regions. This finding has paved the way for proposing biozonations with echinoids, which represents an innovative contribution to the group. An exhaustive cataloging of materials whose specific determination was impossible due to poor preservation has also been performed, highlighting their potential utility in future studies addressing their revision. Furthermore, to facilitate researchers in the taxonomic determination work, comprehensive descriptions, numerous illustrations, several taxonomic keys, and a complete echinological glossary are included, aiming to be useful in future studies requiring knowledge of echinoids. Introductory sections on the disciplines of Paleoecology, Taphonomy, and Ichnology of echinoids are also included, with the aim of establishing a foundation for future research in these areas closely linked to Taxonomy. Finally, the Conclusions section compiles the numerous taxonomic, chronostratigraphic, and paleogeographic contributions derived from this research.

This study offers a comprehensive taxonomic review of 13,530 echinoid specimens from the Eocene of Spain, sourced from 350 sites and 146 localities across the Iberian Peninsula and Balearic Archipelago. It identifies 117 species from 58 genera, many of which had not been described before. The research includes detailed illustrations, taxonomic keys, and an echinological glossary. It also presents new paleobiogeographic data, biozonation proposals, and the identification of stratigraphic indicator species, advancing the understanding of Eocene echinoids. This work serves as a key resource for future research in taxonomy, paleoecology, and related fields.