The European Register of Marine Species (ERMS) project has compiled a list of marine species in Europe and a bibliography of marine species identification guides. ERMS has also surveyed species identification and taxonomic expertise, and the state of marine species collections in Europe. A total of 29 713 species-level taxa were catalogued from European seas. Overall, 90% of the taxon checklists were satisfactory, but non-halacarid Acarina, diatoms, lichens and cyanobacteria were not included, and geographical coverage of the European seas was incomplete for Rotifera and Brachiopoda. Lists that would benefit from further input include (1) those that have not yet been checked by an expert on European fauna, namely lists of the non-epicarid Isopoda, Cephalochordata, Appendicularia, Hemichordata, Hirudinea, Gnathostomulida, Ctenophora and Placozoa; (2) preliminary lists, including some of the above and lists of protists; and (3) lists with many species but which have been reviewed by only a few experts. These gaps are now being addressed in an online version of ERMS (www.marbef.org/data/erms.php). The bibliography of 842 identification guides shows that there are fewer guides for southern European seas, although they contain more species, than for those in northern Europe. Adequate guides for all of Europe’s seas exist only for fishes. New guides are especially needed for the species-rich, but small-sized taxa, such as polychaete, oligochaete and turbellarian worms, and harpacticoid copepods. A database of >600 experts (individuals who stated themselves to be experts) and a subset of these recognised by their peers as being taxonomic experts was established. While there were generally more experts for taxa with a large number of species, there was no correlation between the number of taxonomists and the number of species per taxon; some taxa with thousands of species are studied by relatively few taxonomists. Such gaps in marine biodiversity knowledge and resources must be addressed by funding the production of additional species identification guides.