Sohn, Jae-CheonYponomeutoidea, one of the earliest-branching superfamilies of advanced (ditrysian) Lepidoptera, comprise about 1,800 species worldwide, including notable pests and models of insect-plant interaction. Yponomeutoids were one of the earliest lepidopteran clades to evolve external feeding and to colonize extensively herbaceous angiosperms. Despite the group's economic importance, and its value for tracing early lepidopteran evolution, the biodiversity and phylogeny of Yponomeutoidea have been relatively little studied. Even the monophyly and composition of the superfamily have been in doubt. In this dissertation, the most detailed molecular phylogeny to date for Yponomeutoidea is presented (Chapter 1). The resulting phylogeny is compared to previous morphological evidence, and its implications for evolutionary trends in yponomeutoid host association and biogeography are explored. As a prerequisite to divergence dating in the Yponomeutoidea, which is necessarily based on outgroup fossils as none are known for yponomeutoids, a general summary and overview of the lepidopteran fossil record (Chapter 2) is provided, based a recent, comprehensive catalog of known fossils. For chapter 2, all known lepidopteran fossils have been catalogued with annotations of their preservation, specimen deposition, fossil localities and ages (Chapter 3). As a contribution toward better characterization of yponomeutoid biodiversity, taxonomic reviews are provided for the New World genera Eucalantica and Atemelia (Chapter 4). The molecular phylogeny estimate (Chapter 1) is based on 8-27 protein coding nuclear genes sequenced in 86 Yponomeutoidea and 53 outgroups. Monophyly for Yponomeutoidea is corroborated. Results from different analyses are highly congruent and relationships within Yponomeutoidea are well supported overall. There is strong support overall for monophyly of families (or major parts thereof) previously recognized on morphological grounds, including Yponomeutidae, Ypsolophidae, Plutellidae, Glyphipterigidae, Argyresthiidae, Attevidae, Praydidae, Heliodinidae, and Bedelliidae. The formerly yponomeutid subfamily Scythropiinae are elevated to family rank (Scythropiidae stat. rev.). Host plant family associations of yponomeutoid subfamilies and families are non-random, but show no trends suggesting parallel phylogenesis, and are less conserved than is mode of feeding (e.g. internal versus external). My analyses reveal previously unrecognized tropical clades in several families, and suggest that previous characterization of yponomeutoids as predominantly Palearctic/ Holarctic was based on insufficient sampling.DissertationEntomologySystematic biologyPaleontologyEvolutionFossilLepidopteraPhylogenyPlant-insect interactionYponomeutoidea