Melville, JaneChapple, David G.Keogh, J. ScottSumner, JoannaAmey, AndrewBowles, PhilBrennan, IanCouper, PatrickDonnellan, Stephen C.Doughty, PaulEdwards, Danielle L.Ellis, Ryan J.Esquerre, DamienFenker, JessicaGardner, Michael G.Georges, ArthurHaines, Margaret L.Hoskin, Conrad J.Hutchinson, MarkMoritz, CraigNankivell, JamesOliver, PaulPavón-Vázquez, Carlos J.Pepper, MitzyRabosky, Daniel L.Sanders, KateShea, GlennSinghal, SonalWilmer, Jessica WorthingtonTingley, Reid2022-06-232022-06-231545-7885http://hdl.handle.net/1885/267495Global biodiversity loss is a profound consequence of human activity. Disturbingly, biodiversity loss is greater than realized because of the unknown number of undocumented species. Conservation fundamentally relies on taxonomic recognition of species, but only a fraction of biodiversity is described. Here, we provide a new quantitative approach for prioritizing rigorous taxonomic research for conservation. We implement this approach in a highly diverse vertebrate group—Australian lizards and snakes. Of 870 species assessed, we identified 282 (32.4%) with taxonomic uncertainty, of which 17.6% likely comprise undescribed species of conservation concern. We identify 24 species in need of immediate taxonomic attention to facilitate conservation. Using a broadly applicable return-on-investment framework, we demonstrate the importance of prioritizing the fundamental work of identifying species before they are lost. In order to inform conservation effort, there is urgent need for rigorous taxonomic research to describe species under threat of extinction. Implementation of a new prioritization method identified 282 Australian reptile species needing taxonomic research, of which 17.6% represent undescribed species of conservation concern; this approach could be readily implemented across many faunal groups.JM undertook this work supported by an Australian Fulbright Commission scholarship; S.S. received funding support to undertake research that allowed assessment in the current study from the NSF (DBI-1519732).application/pdfen-AU© 2021 Melville et al.https://creativecommons.org/licenses/by/4.0/2021-06-0110.1371/journal.pbio.30012102021-06-06Creative Commons Attribution License