<p dir="ltr"><i>Philolithus</i> is a genus of large, charismatic darkling beetles (Tenebrionidae) with 18 species found throughout the western portion of North America. Despite many species exhibiting mass emergence events as well as the large size of many of these species, much is unknown about this genus. <i>Philolithus</i> has had a number of taxonomic changes over the last century, a comprehensive revision of the genus was not conducted. In chapter 1, I conducted a species-level revision of <i>Philolithus </i>based on a comprehensive list analysis of external adult morphological characteristics. Based on the results of the study, I proposed the following synonymies, valid names listed in parenthesis “()”<i>;</i> <i>P. pantex </i>(= <i>P. actuosus</i>),<i> P. haruspex </i>(= <i>P. actuosus</i>),<i> P. uteanus</i> (= <i>P. actuosus</i>),<i> P. rugosus </i>(= <i>P. actuosus</i>),<i> P. jageri </i>(= <i>P. actuosus</i>),<i> P. adversus </i>(= <i>P. actuosus</i>), <i>P. reflexus </i>(= <i>P. actuosus</i>),<i> P. porcatus </i>(= <i>P. actuosus</i>),<i> P. opimus </i>(= <i>P. actuosus</i>),<i> P. </i><i>quadripennis </i>(= <i>P. carinatus</i>),<i> </i>and <i>P. sophistes </i>(= <i>P. carinatus</i>). Four new species are also described: <i>Philolithus rotundus </i><b>sp. nov.</b><i>, Philolithus zukomollis </i><b>sp. nov.</b><i>, Philolithus clarki </i><b>sp. nov.</b><i>, </i>and <i>Philolithus dorsoplanus </i><b>sp. nov. </b>Revised taxonomic keys were created for the subgenera of <i>Philolithus</i><i>,</i><i> </i>the species of the subgenera <i>Glyptasida </i>and <i>Philolithus, </i>and the subspecies of the monophyletic subgenus <i>Gonasida</i> based on Dr. Kirby Brown’s unpublished dissertation (<u>Brown, 1971a)</u><i>.</i></p><p dir="ltr">In chapter 2, I performed phylogenetic analyses of <i>Philolithus </i>using targeting enrichment, and Sanger sequenced data for the following mitochondrial loci: 12S, 16S, atp6, atp8, cob, cox1, cox2, coc3, nad1, nad2, nad3, nad4, nad5, nad6 and the nuclear ribosomal 28S gene. Recovered topologies support the monophyly of <i>Philolithus </i>from outgroup Asidini genera, as well as the monophyly and species concepts of the subgenera of the genus.</p>